Xiaomi
Turn Dual Clock On/Off on Lock Screen auf Xiaomi
A second clock on the lock screen shows up when MIUI detects that your set region differs from your current time zone. This usually happens after changing the region in the system settings or after traveling with a SIM from a different country. MIUI compares both time zones and displays them side by side once a difference exists.
This is not a bug. Dual clock is a built-in feature meant to help you keep track of two time zones at once, similar to a travel clock. If you do not need this, you can turn it off, and the lock screen goes back to a single clock.
To manage it, go to Tap on Settings ➔ Tap on Additional settings ➔ Tap on Date & time ➔ Enable or disable Dual clock ➔ Tap on Got it. From the same menu you can also pick which region the second clock represents, so it always matches the time zone you actually want to track.
If the second clock does not disappear even after switching the toggle off, check whether the region setting itself was reverted, since MIUI re-enables Dual clock automatically whenever it detects a mismatch again.