A Free Merge Tool
So who doesn’t want a free merge tool for Salesforce? Yep 100% free, no strings attached.
Cloud Answers have a tool that allows you to merge records just like we did with classic (but in Lightning)!
Get it here: https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3A00000EcyysUAB
Manual Merge
The Manual Merge function is just like the Classic merge feature.
Select the Principal record, then select which values you want from the other records to merge into that principal record:

Bulk Merge
When you have many records to merge and can use one of the available options – Oldest, Newest, or Most recently modified – to select the master record, you can potentially merge hundreds of records at once.

Video
Here’s a quick video of how it works:
Hats off to Ian Cosgrove and the team over at Cloud Answers for this great app. There’s more info in their own blog post below:
Get the app here: https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3A00000EcyysUAB