Prevent multiple instances of the same item ID being appended to an AvatarAppearance

It looks like this was happening when AttachmentsModule.RezAttachments was doing a secondary set of each attachment to update with the asset ID (initially they only have the inventory ID).
However, with multi-attach this was appending a second copy of the same attachment rather than updating the data that was already there.
This commit requires both simulator and service to be updated.
Merge branch 'master' of ssh://opensimulator.org/var/git/opensim