Some of my contacts are displayed twice in my address book — what can I do?
You can get rid of duplicate contacts by deactivating the setting “Show inactive IDs” (Android and Windows Phone) or by activating the setting Hide Stale Contacts (iOS) in Settings > Appearance. Stale contacts are IDs that haven't been used for more than three months. Therefore, it might take a while for contacts to be marked as inactive.
Alternatively, you can delete every contact that is listed multiple times. After each deletion there will be a question whether you would like to exclude the respective contact from future synchronization processes. Please decline that option every time. After you synchronize your address book again, only active contacts (Threema IDs) will be displayed.