Duplicate File Software - Any recommendations please?

I am going through a relative's hard drive which is littered with duplicate files. There are too many to manually remove have already found more than 25 of the of the same photo across a hard drive.

There are over 300,000 files mainly photos which I want to go through quickly and try and put the drive in some sort of order. I have heard Singlemizer in the App Store is good, but has anyone got any solid recommendations from their own experience.

Any help would be really appreciated by both myself and relative.

Many thanks

Matt