Merge With Caution
Users have always had a way of duplicating data, especially back before the dawn of time (Spring 15 - when there was no built in real time duplication prevention). Being able to take two or three accounts or contacts and merge them into one is great feature. On the other hand, losing data while doing it, is not so great. Here are some considerations to consider before merging.
The built in tool for merging accounts and contacts is both a hidden gem and a life saver. The built in merge tool does a lot. It allows you to pick the fields you want to merge, it combines all of the related information but there are some gotchas that you might want to watch out for when merging.
Chatter & Topics
The first thing to consider while merging is Chatter. When you merge two records together, the Chatter posts on both of them do NOT merge. You will be left with the Chatter of only the main record you picked during merging. While this might seem like a small deal, remember that this also includes any files attached in Chatter. Those too will be gone along with any topics assigned.
If you want to make sure you have an archive of all the Chatter, consider using a tool like Compliance Locker (shameless plug), which writes all Chatter to records in custom objects that won't be affected by merging.
Another thing that goes bye bye when merging two records is the field history. This long standing feature lets an Administrator define up to 20 fields that get logged as history related to the record. This is great for auditing who changed what and when but on a merge, only the main record keeps the field history.
If field history is important, consider pulling an archive via the Data Loader, reports or create a custom archive object to re-create the history.
While merging contacts is a little easier because they start in the same account (remember that you have to get them there first), the account name really matters for finding and merging. For example, you might have company called "ABC Carpets" and another duplicate called "Carpets, ABC" the tool won't find them both to allow you to merge. You could search for "ABC" or "Carpets" and one or the other will show up, but not both. The reason is the merge search is pretty basic (too basic?), only really going on the start of the account name, not all of it. To work around this, rename one of the ones you want to merge so they will both show up in the search.