Problems with merging CRSP with Compustat-CRSP merged


For my thesis I would like to merge CRSP with Compustat-CRSP merged. Crsp mainly covers stock market data from exchanges: NYSE, AMEX, NASDAQ. Compustat mainly covers accounting data for public and private companies. Unfortunately I am having trouble by doing so. Here is a small description:

2,201,325 obersevations
31 variables
panel data

Compustat-CRSP merged (annual)
164,280 observations
67 variables
panel data

When I try to merge these datasets one-to-one on key variables (permno and date) in Stata I keep getting the message:

variables permno date do not uniquely identify observations in the using data

Can anyone help me with this?

Has this something to do with fiscal and calender year data?

Kind regards,