You cannot remove, you need to merge them. The duplicate is cause by different UID but it's the same contact. Besides, the Conversations is based on different UID, so removing the old UID will cause that conversations to be deleted? It'll be good to be able to merged the SIMILAR account type, even though it's different UID.

Plus, if Facebook Chat decided to use back the old UID, then it's not wise to remove the old UID.

Facebook Chat should use a fixed and standardize UID. The UID shouldn't change...