Is there such a thing as double jeopardy protection, similar to the 7th Amendment, for the military?

you can not get court marshaled twice for the same offence however you can be punished and then receive other administrative actions like a chapter. A chapter is administrative not punitive.

"can a soldier be involuntarily separated under Chapter 14 if they have already received an Article 15 for the same offense?"

YES, One is Military Justice, the other is administrative action.

You could not receive an ART 15 for theft, and then later CM for theft of the same thing.

If you were caught with a stolen piece of property, and given an ART 15 for it, and it was later discovered you had burglarized a home to steal the property, assaulted the owner... then Im not positive but I "think" you would be safe from the crime of property theft, but could be charged for the other crimes committed

a discharge has nothing to do with double jeopardy. double jeopardy is protection against being prosecuted for the same crime twice, which you do have. they can not ART. 15 and then court martial you for the same offence. But they can NJP you and then discharge you.