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Your popups could be legit, or could be evidence that you have been prowling on the "wrong" kind of (prøn) sites.
Run a Malwarebytes scan. You may discover that you have downloaded some kind of malware or adware.

Many folks here would say that if you already have Flash installed, that you should remove it.
But, maybe you have a need - so go to an actual site where you can download your own flash updater.
This would be OK - if there's anything about Flash that is still OK, anyway.
 
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