diff --git a/copyparty/httpcli.py b/copyparty/httpcli.py
index bc1f607f..68f1a64a 100644
--- a/copyparty/httpcli.py
+++ b/copyparty/httpcli.py
@@ -855,63 +855,67 @@ class HttpCli(object):
         response = x.get()
         chunksize, cstart, path, lastmod = response
 
-        if self.args.nw:
-            path = os.devnull
-
-        if remains > chunksize:
-            raise Pebkac(400, "your chunk is too big to fit")
-
-        self.log("writing {} #{} @{} len {}".format(path, chash, cstart, remains))
-
-        reader = read_socket(self.sr, remains)
-
-        f = None
-        fpool = not self.args.no_fpool
-        if fpool:
-            with self.mutex:
-                try:
-                    f = self.u2fh.pop(path)
-                except:
-                    pass
-
-        f = f or open(fsenc(path), "rb+", 512 * 1024)
-
         try:
-            f.seek(cstart[0])
-            post_sz, _, sha_b64 = hashcopy(reader, f)
+            if self.args.nw:
+                path = os.devnull
 
-            if sha_b64 != chash:
-                raise Pebkac(
-                    400,
-                    "your chunk got corrupted somehow (received {} bytes); expected vs received hash:\n{}\n{}".format(
-                        post_sz, chash, sha_b64
-                    ),
-                )
+            if remains > chunksize:
+                raise Pebkac(400, "your chunk is too big to fit")
 
-            if len(cstart) > 1 and path != os.devnull:
-                self.log(
-                    "clone {} to {}".format(
-                        cstart[0], " & ".join(unicode(x) for x in cstart[1:])
-                    )
-                )
-                ofs = 0
-                while ofs < chunksize:
-                    bufsz = min(chunksize - ofs, 4 * 1024 * 1024)
-                    f.seek(cstart[0] + ofs)
-                    buf = f.read(bufsz)
-                    for wofs in cstart[1:]:
-                        f.seek(wofs + ofs)
-                        f.write(buf)
+            self.log("writing {} #{} @{} len {}".format(path, chash, cstart, remains))
 
-                    ofs += len(buf)
+            reader = read_socket(self.sr, remains)
 
-                self.log("clone {} done".format(cstart[0]))
-        finally:
-            if not fpool:
-                f.close()
-            else:
+            f = None
+            fpool = not self.args.no_fpool
+            if fpool:
                 with self.mutex:
-                    self.u2fh.put(path, f)
+                    try:
+                        f = self.u2fh.pop(path)
+                    except:
+                        pass
+
+            f = f or open(fsenc(path), "rb+", 512 * 1024)
+
+            try:
+                f.seek(cstart[0])
+                post_sz, _, sha_b64 = hashcopy(reader, f)
+
+                if sha_b64 != chash:
+                    raise Pebkac(
+                        400,
+                        "your chunk got corrupted somehow (received {} bytes); expected vs received hash:\n{}\n{}".format(
+                            post_sz, chash, sha_b64
+                        ),
+                    )
+
+                if len(cstart) > 1 and path != os.devnull:
+                    self.log(
+                        "clone {} to {}".format(
+                            cstart[0], " & ".join(unicode(x) for x in cstart[1:])
+                        )
+                    )
+                    ofs = 0
+                    while ofs < chunksize:
+                        bufsz = min(chunksize - ofs, 4 * 1024 * 1024)
+                        f.seek(cstart[0] + ofs)
+                        buf = f.read(bufsz)
+                        for wofs in cstart[1:]:
+                            f.seek(wofs + ofs)
+                            f.write(buf)
+
+                        ofs += len(buf)
+
+                    self.log("clone {} done".format(cstart[0]))
+            finally:
+                if not fpool:
+                    f.close()
+                else:
+                    with self.mutex:
+                        self.u2fh.put(path, f)
+        finally:
+            x = self.conn.hsrv.broker.put(True, "up2k.release_chunk", ptop, wark, chash)
+            x.get()  # block client until released
 
         x = self.conn.hsrv.broker.put(True, "up2k.confirm_chunk", ptop, wark, chash)
         x = x.get()
diff --git a/copyparty/up2k.py b/copyparty/up2k.py
index b70129c8..a80168fb 100644
--- a/copyparty/up2k.py
+++ b/copyparty/up2k.py
@@ -494,6 +494,7 @@ class Up2k(object):
                 if bos.path.exists(path):
                     reg[k] = job
                     job["poke"] = time.time()
+                    job["busy"] = {}
                 else:
                     self.log("ign deleted file in snap: [{}]".format(path))
 
@@ -1256,6 +1257,7 @@ class Up2k(object):
                             "at": at,
                             "hash": [],
                             "need": [],
+                            "busy": {},
                         }
 
                 if job and wark in reg:
@@ -1338,6 +1340,7 @@ class Up2k(object):
                     "t0": now,
                     "hash": deepcopy(cj["hash"]),
                     "need": [],
+                    "busy": {},
                 }
                 # client-provided, sanitized by _get_wark: name, size, lmod
                 for k in [
@@ -1444,6 +1447,11 @@ class Up2k(object):
             if not nchunk:
                 raise Pebkac(400, "unknown chunk")
 
+            if wark in job["busy"]:
+                raise Pebkac(400, "that chunk is already being written to")
+
+            job["busy"][wark] = 1
+
         job["poke"] = time.time()
 
         chunksize = up2k_chunksize(job["size"])
@@ -1453,6 +1461,14 @@ class Up2k(object):
 
         return [chunksize, ofs, path, job["lmod"]]
 
+    def release_chunk(self, ptop, wark, chash):
+        with self.mutex:
+            job = self.registry[ptop].get(wark)
+            if job:
+                job["busy"].pop(wark, None)
+
+        return [True]
+
     def confirm_chunk(self, ptop, wark, chash):
         with self.mutex:
             try:
@@ -1463,6 +1479,8 @@ class Up2k(object):
             except Exception as ex:
                 return "confirm_chunk, wark, " + repr(ex)
 
+            job["busy"].pop(wark, None)
+
             try:
                 job["need"].remove(chash)
             except Exception as ex:
diff --git a/copyparty/web/up2k.js b/copyparty/web/up2k.js
index 5375c4ab..e8b50876 100644
--- a/copyparty/web/up2k.js
+++ b/copyparty/web/up2k.js
@@ -1843,7 +1843,8 @@ function up2k_init(subtle) {
                 st.bytes.uploaded += cdr - car;
                 t.bytes_uploaded += cdr - car;
             }
-            else if (txt.indexOf('already got that') !== -1) {
+            else if (txt.indexOf('already got that') + 1 ||
+                txt.indexOf('already being written') + 1) {
                 console.log("ignoring dupe-segment error", t);
             }
             else {
@@ -1851,6 +1852,9 @@ function up2k_init(subtle) {
                     xhr.status, t.name) + (txt || "no further information"));
                 return;
             }
+            orz2(xhr);
+        }
+        function orz2(xhr) {
             apop(st.busy.upload, upt);
             apop(t.postlist, npart);
             if (!t.postlist.length) {
@@ -1872,9 +1876,11 @@ function up2k_init(subtle) {
                 if (crashed)
                     return;
 
-                toast.err(9.98, "failed to upload a chunk,\n" + tries + " retries so far -- retrying in 10sec\n\n" + t.name);
+                if (!toast.visible)
+                    toast.warn(9.98, "failed to upload a chunk;\nprobably harmless, continuing\n\n" + t.name);
+
                 console.log('chunkpit onerror,', ++tries, t);
-                setTimeout(do_send, 10 * 1000);
+                orz2(xhr);
             };
             xhr.open('POST', t.purl, true);
             xhr.setRequestHeader("X-Up2k-Hash", t.hash[npart]);