you might have tried number of times to question him/her .."that is my idea and why you have stolen my idea.."?? But you really can not ask and you feel embarassing to ask ..but you can tolerating by keeping quite...

so lets analyse this...

first of all It's offensive, so the first question is what exactly do you mean by taking credit, is it that you're not getting a thank you from your manager, is it not showing up in your annual performance appraisal?

Credit is a very sort of political tricky thing you have to get clear about what it is you're after.

" Well remember that in collaborative work there's a lot of ideas flying around and sometimes people forget kind of who gave what anyway and that's very natural. So, you know, there is a saying "people remember what is said but forget who said it". And so you can walk the halls saying I'm doing a great job this month and by the end of the month everyone forgot that it was you that said it, they just know you did a great job. And that's not exactly true but the idea is don't get caught up in the line by line score card keep doing the work, be happy with that."

Psychologically..every humanbeing responds or reacts same way as others...

Hence if there is any action which needs a good ideas ..all most all your coworkers get the same idea...

BUT remember here the "Timing" is important...when and how fast ..how qucikly your ideas are spreading and publishing...if you maintain this ..then no one will catch you and your ideas..and ur rewards..

In summary, again,

it's not a score card do the work, contribute your stuff, make sure you're getting some credit overall but don't worry about one individual idea.
do a great job, and make sure your work is noticed but don’t worry about getting credit for every single idea.