Create document via copy fully overrides existsing with the same name

When document is created via copy, it does not warn if document with same name already exists and completely overrides it, including history - so that there's even no ability to revert the override.

If document with the same name already exists, the system should either
a) warn user about the conflict
b) preserve document history by merging 2 histories

(this bug was discovered in conjunction with XE-1209