Board Thread:The Panopticon/@comment-4028641-20151101035254/@comment-26469787-20151118054050

Except there isn't. Bonnie was given her own name, given individual opinions and numerous lines of dialogue, and had a leading role and plot function within her story.

"Zygon Osgood" was given none of those things, and may not even have existed as a separate entity post-DotD, as per the Doctor's statements. Assuming differently would be speculative in the extreme.

If, and only if, we are again presented with Osgoods of unclear identity, we can and almost definitely should merge all three. But until then, there is no in-universe sourcing that ever makes unclear who Bonnie is. She is always a separate entity to "Osgood", even during the brief time she takes on Petronella's appearance.