This shouldn't happen, but rename the file mentioned in the error on the other computer (by right-clicking on the attachment title in the right-hand pane and making sure "Rename associated file" is checked) and resync, and then resync this computer.
Also, if you can tell us where the parent item came from, we can try to reproduce and fix this.
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Also, if you can tell us where the parent item came from, we can try to reproduce and fix this.