Hello, everyone

I have an employer-employee database and need to keep only the individuals that have at least one colleague considering the Firm_id variable by year, but I don't know how to do this in Stata. My dataset is like this:
Code:
Id Firm_id Year
1 50 2010 
1 50 2011
2 50 2010
2 50 2011
3 22 2010
3 22 2011
4 22 2010
4 20 2011
In the case above, I would keep only the individuals corresponding to the Id 1 and 2 because they are in the same firm in both of the years in the sample and Id 3 and 4 for 2010.

The output I'm looking for is like:
Code:
Id Firm_id Year
1 50 2010
1 50 2011
2 50 2010
2 50 2011
3 22 2010
4 22 2010
Any suggestions on how to perform this in Stata?

Regards,