if i buy a full game in the app store can i also get the full game on the other devices connected to me family share plan without being charged twice for the full game.

In-app purchases aren't shareable to other accounts in family sharing : What types of content can I share with Family Sharing? - Apple Support

What you can't share

• Songs added to iTunes Match that weren't purchased from the iTunes Store.
• Individual or student Apple Music memberships and other subscriptions, such as magazines and newspapers.
• In-App purchases.
• Items that are no longer available on the iTunes Store.
• Items that you or another member of your family group have hidden. Learn more about hidden purchases.
• Some apps from the App Store. Find out if your app is shareable.
• Ringtones.

If you mean the app itself is the purchase, and you buy it on your account, then it should show as redownloadable on your account on your other devices. If it's an in-app purchase in the app to unlock the full game then it might depend upon the game/developer