If is stolen I would get so mad but if someone really though with the same mentality as me and made a move before me i would offer my collaboration and do the idea together
Collaborating would not be easy because that team or company has already launched before you and may become the market leader if such projects do not exists. So chances become slim as they themselves would like to have the market share rather than sharing with others. Yes if such projects do already exists than its better to have more on boards so that your project become more effective than the existing ones.
This would be only on slim chances of cases where if they would able launch out without any competition or do have the unique concept which is originally yours and did able to get attention on the community then market share would really be generally belonged to them not yours and you will end up to be the one who would copy on the idea in the end because people do already see it first into what they do saw recently.