I have the following situation:
clone(Y) from a main repository(X), because there were many people working on Y we didn't do any rebase but only merges. When we want to deliver(push) Y to X we would like to do a rebase in order to have things nice and cleanThe problem is that when doing rebase we are asked to do all the merges that we already did in the previous merge steps. Is there a solution to this, beside the one that means actually re-doing the merges?
I expected it to be pretty straightforward since we already solved the conflicting merges.
Rebasing to get a "clean" history is overrated. The best way if you want to preserve history is just to do the merge instead of a rebase. That way if you ever need to go back to a revision, it is exactly the same as the one you tested during development. That also solves your issue about the previously solved merge conflicts.
If you don't care about preserving history, you can create a new branch off of master, check it out, then do a git read-tree -u -m dev to update your working tree to match the dev branch. Then you can commit everything into one big commit and merge it into master as normal.