There are 4 cases when you may have this question:
1) when you upload a list of keywords using CSV
2) when you add keywords from one ad group to another
3) when you add search terms as keywords to another ad group
4) when you add keywords from the Keyword Planner to your ad groups
The general rule is if you try to upload the same keywords into the same group, you will see an error message and SearchAdsHQ will not allow you to add duplicates to the same ad group. However, you can add them to different groups, so you need to be careful with this not to have the same keywords across multiple ad groups. In the future, we will improve the deduplication mechanism to prevent this, for right now please bear this in mind.
When you try to add them, you will get an error message. You can download an error report from here to see what keywords are duplicates. Tick "add only keywords without errors" in order to add the keywords.