I wouldn't be overly concerned about inflated scores from free online tests, since they typically do not have the actual test items (which are copyrighted), but the most straightforward way to manage those types of validity questions is to tell your examiner upfront about all of the unofficial and official testing you've done in the past.
Most (legitimate) tests have a valid retest interval (meaning how long you should wait between retests of the identical measure) of at least six months (for academic-type tests), 12 months (many neuropsych tests) or 24 months (for cognitive tests).
The main thing you can do at this point is to stop taking online tests, and to be honest and thorough with your examiner.
