If you have contacts in your Telegram account which you don't recognize, you have most likely logged in on another person's phone – and uploaded their contacts to your account by accident.
You can wipe all previously uploaded contacts from Telegram and then re-upload them from your current device. On your mobile device go to Settings > Privacy and Security > Data Settings > Delete Synced Contacts (using official apps for Android or iOS). To re-upload your up-to-date address book, make sure the Sync Contacts toggle is turned on.
Note: If you need to use someone else's device to log into your account, make sure to disable Sync Contacts when entering your phone number on the login screen. This will prevent the local phonebook from being imported to your account. Telegram will also warn you before importing an unusually large number of new contacts.