There are several ways to manage or prevent the creation of duplicate records:
The Merge Wizard allows you to merge up to three Account, Contact, or Lead records. You must click on the “Merge object” button and select the records that seem similar and that you want to merge. Then, via the Wizard, select the values to retain;
It is possible to prevent users from creating duplicates for Accounts, Contacts, Leads, and Custom Objects. For this purpose, you need to create Matching rules and Duplicate rules. The new record is compared to the existing Salesforce records and via the matching rule it can be identified as a duplicate. The duplicate rule could block users from saving the possible duplicate record or allow them to save it anyway.
NB: the implementation of matching and duplicate rules is recent (2015), they function properly as complement to the possible Appexchange apps already installed.
If the prevention of duplicates is based on a different object, Appexchange solutions exist.