Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 265316)
+++ channels/chan_sip.c	(working copy)
@@ -23775,11 +23775,25 @@
 
 	while ((mbox = context = strsep(&next, ","))) {
 		struct sip_mailbox *mailbox;
+		int duplicate = 0;
 
-		if (!(mailbox = ast_calloc(1, sizeof(*mailbox))))
+		strsep(&context, "@");
+
+		/* Check whether the mailbox is already in the list */
+		AST_LIST_TRAVERSE(&peer->mailboxes, mailbox, entry) {
+			if (!strcmp(mailbox->mailbox, mbox) && !strcmp(S_OR(mailbox->context, ""), S_OR(context, ""))) {
+				duplicate = 1;
+				break;
+			}
+		}
+		if (duplicate) {
 			continue;
+		}
 
-		strsep(&context, "@");
+		if (!(mailbox = ast_calloc(1, sizeof(*mailbox)))) {
+			continue;
+		}
+
 		if (ast_strlen_zero(mbox)) {
 			ast_free(mailbox);
 			continue;