CSV Import: How not to replace existed users and avoid doublicate?

Hello. I have 2 sites:
1) EasySocial based communuty
2) Simple Joomla articles site
that I want to merge users. Need to move Joomla's users to Community's users.

So I need first to install EasySocial on Simple Joomla site and export CSV file.
Then I just need to Imort file to communuty?
Sounds easy

But communuty and Joomla has about 70% common users (because few years ago I made communuty's copy > it become separated Joomla site). So 70% of users has the same usernames, ID and may be Emails.

During moving users from Joomla > communuty I'm interesting:
1) not to copy allready existed users (avoid doublicate)
2) not to replace allready existed users

Does system respect it? How to prevent it?

Thanks.