Hi. If you want to detect the same face (with same ID) you'll need to do a face recognition work too.
There are algorithms that detects the face (haar cascade for example), but if the face exits of the image and returns, it needs the face recogniton... Did you understand?
I work with opencv in C, C# and Java (Desktop and Android Apps).
I have an app that detects face and I'm working in face recognition right now.
I would like to know this job for you. If possible, send me more details about the project. After that I can rebid the job.
Regards,
André