Anytime you have duplicate records in Access, you run the risk of having corrupt data in your reports, because databases rely on normalized data for all tables and relationships. Normalized data means that entered data exists in only one table, and is utilized everywhere else throughout the database. This does not mean that you will not see the data in other places, only that you will enter it in one table.
Once records are entered, duplicate records can be easily found and removed by running queries that look for just that type of thing. The Query Wizard gives us an easy way to create these types of queries, so that you do not need to worry about setting them up from scratch.
Check out this video that will show you how to do it: