To go hard in what you do, you can get it in with females, money, hoop'n etc..

To fuck bitches

sex.

To have a fun or entertaining time with, usually as compensation for a period of extended separation between two or more people.

to be successful at something difficult

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.