Need some more information:
1. What exact Mac model do you have?
2. What do you mean by "application crashes"? Is it more than one application?
3. I assume you upgraded to Mojave "in place", ie, basically let Mojave over write what you had on your Mac already. Correct? Also, what Mac OS were you previously using?
4. Did you insure than all your third party applications (ie, non-Apple ones) were compatible with Mojave? THis link can help with that:
5. A newer version of Mojave, V10.14.3, was released last week. You can get the OS 10.14.3 Combo Updater from here:
Just make sure you have a backup. You are making backups, correct?
Regarding duplicate contacts, and assuming you are using Apple's Contacts application, this link should help you with that:
Again, though, make sure you have a backup first.