Queries :: Merge Two Tables

I have a database with the following tables:

- data
- datamodifications
- mergeddata

The "data" table is imported from a large excel file (about 5000 rows and 40 columns) and needs to be updated about once a year. The idea is to use the "datamodifications" table to store all modifications and then merge "datamodifications" and "data" into "mergeddata", so when i have to update the "data" table next year i can just remerge it with my modifications. My problem is that i cant merge the two without getting dupliace entries. I have tried with this query but it gives me duplicate entries:

Code:
SELECT col1, col2, col3
FROM data
UNION
SELECT col1, col2, col3
FROM datamodifications
ORDER BY col1;

How can i remove the duplicate entries (or is there a better way than actually writing SQL to do what i want do to?