It is possible that a contact could have a tag added (and commence receiving a drip campaign), then removed, and then re-added. While uncommon, you can prevent this contact from restarting a drip campaign within a short period of time, by adding the condition of "If Sent Email" before the first email.
Here's how:

• Set your recipients, choosing to either:
  
  • Select particular tags and/or audiences, to filter your reach before adding any triggers or conditions.
  • Or, set to Send to all contacts, to only use the trigger and condition as your contact filters.
• Within Tag Changed, select the tag to trigger the campaign by, e.g.: Buyer.
  • Set the tag change, e.g.: Added.
• Then, add a new module, and choose the condition of If Sent Email. 
  • Set the condition to Was not sent.
  • Select the email you don't want them to receive again, e.g.: Buyer email #1
  • Optionally set a timeframe, to only exclude contacts targeted more recently, e.g.: Within the last 20 Days.
• Add a new module, and choose Send Email.
  • Select the same (first) email for your campaign, e.g.: Buyer email #1

With these example settings, if contact "Joe" has the tag of "Buyer" added, and he has not been sent "Buyer email #1" within the last 20 days, Joe will receive "Buyer email #1". If the tag is removed, and re-added 19 days later, Joe will not receive Buyer email #1 again.*

* Be aware that using this condition will also affect any other campaigns sending the same "Buyer email #1" email.

* Learn more about how to use Tags in a campaign