error handling for duplicate torrents

index.js

@@ -168,8 +168,17 @@ WebTorrent.prototype.download = function (torrentId, opts, ontorrent) {
   if (ontorrent) self.on('torrent', clientOnTorrent)
 
   torrent.on('error', function (err) {
-    self.emit('error', err, torrent)
-    self.remove(torrent)
+    var alreadyExistsMessage = 'There is already a swarm with info hash'
+    if (ontorrent && err.message && err.message.substr(0, alreadyExistsMessage.length) === alreadyExistsMessage) {
+      self.torrents.splice(self.torrents.indexOf(torrent), 1)
+      torrent = self.torrents.filter(function (_torrent) {
+        return torrent.infoHash === _torrent.infoHash
+      })[0]
+      process.nextTick(clientOnTorrent.bind(null, torrent))
+    } else {
+      self.emit('error', err, torrent)
+      self.remove(torrent)
+    }
   })
 
   torrent.on('listening', function (port) {

test/duplicates.js

@@ -0,0 +1,45 @@
+var fs = require('fs')
+var test = require('tape')
+var WebTorrent = require('../')
+
+var leavesBook = fs.readFileSync(__dirname + '/content/Leaves of Grass by Walt Whitman.epub')
+
+test('client.seed followed by duplicate client.add', function (t) {
+  t.plan(2)
+
+  var opts = {
+    name: 'Leaves of Grass by Walt Whitman.epub'
+  }
+
+  var client = new WebTorrent({ dht: false, tracker: false })
+  client.seed(leavesBook, opts, function (torrent1) {
+    client.add(torrent1.infoHash, function (torrent2) {
+      t.equal(torrent1.infoHash, torrent2.infoHash)
+      t.equal(client.torrents.length, 1)
+    })
+  })
+})
+
+test('client.seed followed by duplicate client.add, twice', function (t) {
+  t.plan(4)
+
+  var opts = {
+    name: 'Leaves of Grass by Walt Whitman.epub'
+  }
+
+  var client = new WebTorrent({ dht: false, tracker: false })
+
+  client.seed(leavesBook, opts, function (torrent1) {
+    client.add(torrent1.infoHash, function (torrent2) {
+      t.equal(torrent1.infoHash, torrent2.infoHash)
+      t.equal(client.torrents.length, 1)
+    })
+  })
+
+  client.seed(leavesBook, opts, function (torrent1) {
+    client.add(torrent1.infoHash, function (torrent2) {
+      t.equal(torrent1.infoHash, torrent2.infoHash)
+      t.equal(client.torrents.length, 1)
+    })
+  })
+})