So I did a merge through cmd which did not end up well, so I undid that CL (through visual client), now if I try to merge the 2 streams again I get a window saying Integration errors: //project_stream/main/... - all revision(s) already integrated. Does it mean there is no way to do the merge again?
If you undo an integration via p4 undo and the dm.integ.undo configurable is set you can just re-do an integration exactly as you tried to do.
(p4 help undoc)
dm.integ.undo 0 Enable re-integration of undone changes
If those two conditions can't be met (i.e. you did the undo with a version of P4V that doesn't use the native undo command and/or you can't get your admin to flip that configurable setting), you have to re-integrate with the -f flag.
The -f flag forces integration without regard to previous history, which allows you to redo an integration -- however, you need to explicitly tell it which revisions/changelists to integrate, and/or take extra care during the resolve process. Look carefully at the integration records from your original changelist to figure out what the earliest source changelist would have been, and do:
p4 integ -f source/...@SOURCE_START,@now target/...
The revision before SOURCE_START will be used as the base during the resolve process.