I'm still working with support, but I'm posting in case anyone else sync problems that could be due to this issue.
The bug occurs when I use continuous stream or copy features that have photo attachments; in other words, "Like the last one" functionality.
- My features are displaying the same attached photos. Sync Fails.
- If i delete one they both lose all photos. Sync Still Fails.
- If I delete all features. Empty Sync Succeeds.
Here are images of the 1st and 5th test feature i took. for each feature i copied the previous and attached a photo via camera. but the 1st feature shows the attachments collected on the later features.


Here is what happens on the 1st sync attempt

During second attempt, the progress bar quickly moves to 99% but hangs indefinitely.
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Details:
10.3 version of Collector on 2 different Samsung Tab 3 7" and a Samsung S5
Reproduced in 2 different web maps with same hosted layers
Arcmap's Copy Runtime Geodatabase to File Geodatabase tool produces features with duplicate attachments.
Could not reproduce in different hosted layers in separate web map. The attachments were not carried over when copying a feature
Initial download for offline mode has no issues
I'll update when this gets resolved.