@steenth
I'm not sure which version of 1Password you are using. Can you indicate version of the Desktop app?
Generally, go to the Vault where the contents are, that you want to delete, then do Control - A to select All. Then Right-click the selected items, and select Delete.
Be sure that you are deleting only the items you want deleted. It's unclear which vault your items were imported into, nor if you have other items that you don't want deleted. So be cautious here.
Can I change the default vault?
This depends on which version of 1Password you are running, hence the question above.
Can I remove duplicates?
This also depends on which version of 1Password you are using - version 7 has a tool to help remove duplicates, version 8 does not yet.