to scheme or get over on someone or to try and hustle someone

The verb GET is a synonym for to obtain. Right.
To GET GET would therefore mean to OBTAIN OBTAIN.
Now the verb GET GET is a slightly downgraded, less elderly offensive, and actively 007ish combination for GETTING SOMEONE or even, with the appropriate low rise intonation, GETTING LAID.

Now spread the gospel (when you're not get getting, that goes without saying.)

A positive statement to pump someone up like a cheerleader. To achieve a goal. Reinforcement on the repetitive notion to achieve maximal greatness at the task at hand.

Phrase encouraging whomever is listening to go ahead and move forward, doing whatever it is one is talking of doing without haste.

Getting in is before you do anything sexual or romantic, it's the getting close and talking before hand.

Get by = manage

When someone has to manage sth or a situation

The act of sexual intercourse.