Ideally, you should dedupe all records or merge them together, if appropriate.
The impact will be the potential to send email to the wrong accounts or Contacts if they have dupes across Accounts or the Owners are not aligned.
The other issue is data quality.
Another issue is that you have one person with several entries and email addresses, they unsubscribe from ONE of those, but the others continue to be sent email and that person gets mad at you.
There could be good reasons to have several Accounts for Cisco if they are at different locations or for different sales territories. Ideally they'd be in an Account Hierarchy, but that only affects SFDC.