To activate a rule for unique passwords:

1. On the left panel of your admin dashboard, head to People > Settings.

2. Click on the App Settings tab.

3. On the left of the section, click Password Settings.

4. Toggle Enforce Password History Policy? switch ON (green).

5. Select the number of unique, new passwords that must be used by an employee before an old

    password can be reused. For example if you select 2, employees will have to use 2 new

    passwords after their original password expires, before they can reuse the original one.

Read more:

How to set an expiration date for passwords