debian_pipeline with enable_make_signed_source and enable_upload creates duplicated mergechanges tasks

This is the second part reported in #796 (closed)

I have a (private) debian_pipeline with enable_make_signed_source, that is meant to be uploaded with binaries by using enable_upload, upload_include_binaries, and upload_merge_uploads. For some reason, multiple duplicated mergechanges tasks get created, with some of them (all but the first) apparently being created some time after the initial set of tasks.

The fix in commit 3c698b65 apparently didn't fix the duplicated tasks.