When using composer in large groups, your composer.lock file will often be the source of merge conflicts. This can happen for a multiple reasons, but generally it happens when the branch you're merging in has updated to a different version of the same library. I used solved this issue was by deleting my lock file and running composer update. As long as everything was setup right, what could go wrong?

The other day I came across a better solution.

git checkout --theirs composer.lock

During merges git offers --theirs and --ours flags to checkout command. These flags allow you retain the file changes for either the branch being merged --theirs or to retain your changes --ours.

I use the --theirs flag to "reset" the lock file to the state of the branch being merged in. I run composer update vendor/package for the packages that I know I updated.