Monthly Data Refresh - Overlay Data

Greetings.

I have a monthly process that needs to add the new rolling dataset every month. This process also involves keeping historical data frozen within the data set. Currently the process is:

1. The Historical File is updated 1 time per year. This data is the “frozen” data.
2. The monthly file is updated, on a rolling basis, and loaded into the system each month.
3. As I import the monthly data I then append the Historical data to it into a table called Main.

My issue is, how do I overlay completely the existing monthly data table? Currently it is just adding the new data to the existing table. This is providing wildly inflated counts.

Any ideas?

Thanks