How to replace duplicated multi-line groups, e.g. from git & diff3

I'm rebasing legacy code, and finding lots of conflicts caused by scripts -- usually code formatters. The changes are simple and predictable, so I can trivially re-run the script to apply the changes to the code -- which usually leaves me with three identical "snippets" surrounded by git's diff markers. How can I write a script to find three matching snippets, with their diff markers, and convert them to a single snippet? Example: git rebase produces this:

>>>>>> 0cfd85b8e3... Beautify.

Re-running the formatter gives this:

>>>>>> 0cfd85b8e3... Beautify.

which I would like to convert to this:

ACC
    BCC

I've tried many, many things including grep (it struggles with new-lines) and pcregrep (I struggle with back-references/capture-groups). Any ideas?