Merging

You're probably not going to use anything but a +Atk Reinhardt so the other one's use is already gone (only in very niche cases would he have any use).

Now you have 3 options
1. Just keep him in your inventory
2. Merge to give +Atk Rein 1 extra attack and hp
3. Inherit his skills to another unit.
-Dire Thunder is uninheritable
-Blazing Thunder useless
-Vantage 3 can be obtained from a 4* Lon'qu and 4* Reinhardt so salvaging a 5* isn't a good deal
-Goad Cavalry is good, but most times Hone Cavalry is preferred.

So skill inheritance isn't that good on Rein. Merging is a good choice in this case, but is irreversible, so that's entirely up to you.

Also don't forget that the game seems to head to a direction where you can't really use duplicate units.

Squad Assault, Arena Assault are game modes where you can't use the same character twice, even if it is not the same unit.
So duplicates are not that good of an idea !

Assuming you're F2P, 5* merging is usually not optimal due to the expense of 5* exclusive skills. With Reinhardt, Dire Thunder is not inheritable and Vantage 3 is relatively common and available as a 4* skill on him and Lonqu. However, Goad Cav is a 5* exclusive and has become more useful with the influx of units that ignore movement-specific Hone/Fortify buffs.

So, I would suggest that you find a good recipient of Goad Cav (really anyone you use frequently in a cavalry comp). If you don't ever see yourself running a cavalry comp, it's fine to merge.