I am in a team of 3 senior developers. In our current assignment we were supposed to analyze a certain project and identify key areas to work on. After that, each of us was supposed to take one area and focus on implementing certain amount of tasks from that area with our own respective teams.
From us three he (colleague A) is the one with least knowledge about the project. He did not make any choice of his "own" area to focus on during the meeting. The other colleague and I did choose corresponding areas and it was agreed upon during the meeting.
The same day, after the meeting, I have found out that colleague A went talking to people behind my back about tasks from my chosen area. This has led to him being invited to a meeting where we both would be briefed on the subject.
I have found out that right after meeting, he secretly had a discussion with my manager where he got approval for doing exactly that.
So now, instead of working on his own, he "invited himself" behind my back to interfere with my work.
The manager told me that although that area is mine, for a "pre-discussion/pre-briefing" we might have multiple people sitting in meetings (including him) to "be able to understand all details" of what has to be done.
That colleague has a record in my projects of hijacking/softly taking-over our common work in the past or position himself as the number one contributor, even if our efforts were around 50/50.
He never did that right away. Rather he did it gradually with time. I am afraid that this is another case of such behavior now.
What can be done in such cases against this behavior? The manager is a very "hands-off" type with high turnover, cases of blind favoritism and bias. So, I have no idea how to react now.