Hi there,
I’ve finally managed to get most of my people to use Kontalk.
So far so good, but one of them uninstalled Kontalk without exporting his keys first.
After re-installation he can’t get Kontalk to work, because it says “Number already exists”, and he has no keys from his first installation.
What to do?
Thanks for your great work! 
I’m sure your friend gets a warning his Phonenumber is already registered in the Kontalk network.
And if he continue the registration his current key will revoked or invalid.
That’s correct. Because a key can for security reasons never restored from Kontalk. If you lost your key you always have to create a new one.
So he just have to finish the registration.
His old key will be invalid and the new key will sendet to his contacts as soon there contact lists will be automatically refreshed. This may take a few houres.
To get immediately the new key you can refresh your contact list manually.
And all his contacts have to accept the new key.
That’s all.
Thanks @webratte for the very thorough answer. I want to add one more thing that happens when the account is replaced with a new key. For security reasons, the server will reset authorizations between the user and all of his/her contacts, so he/she will have to request chat invitation to everyone again.
Thank you for your answers and sorry for my late reply.
Finishing the registration process did indeed solve the problem.it’s beyond me why he had insisted to have done everything completely.
Thanks again!