Yes, a contact can be stored in multiple groups. If all of those groups are added to a campaign, the contact will only receive one email (they will not receive an email for each group they are included in).
Click the down arrow next to a contact's name and select Edit to modify which groups they are stored in.
To find out how to remove contacts in more than group or remove duplicates see here
If you have selected more than one contact group to send a campaign to, and the contact is in more than one of them, once you deselect a contact group that the contact is in, the contact will be removed as a recipient to the entire campaign.
To reverse this, you can re-select the contact groups that you would like to send the campaign to.