Creating suppression lists, including duplicate prevention across your team or within your account is described in detail in this post.

In the case where you may be managing multiple accounts (ie - you run an agency or consultancy), you may need to create Teams within Teams so you can have suppression lists and/or duplicate rules applied within a specific team and not applied across all teams/users.

To set this up, make sure you invite the Team Admin (ie - your client's main point of contact) and their coworkers to join your Team so you can manage billing.  Once they've accepted the invite and granted you Account + Campaign permissions, you'll be able to access their account.

If you go to your Suppression List and apply rules (like prevent team duplicates) all 20 of your clients will be subject to the rule.  This can create a problem in the scenario where 2 or more different clients need to contact the same person.  If you have Team Duplicate prevention turned 'ON' then once 'John from Client 1' emails 'prospect Ryan O'Donnell', Susan from Client 2 (who also has Ryan in her campaign) won't be able to email Ryan.

To prevent this from happening, if any of your 5 clients have specific requirements for creating suppression lists or if they want to prevent duplicates across their coworkers, you'll need to Create a new Team from within your client's accounts (ideally the client admin/main point of contact).

Provide them with these instructions or 'Switch Accounts' into their account to set this up.