Q: new duplicate photo remover imac with el capitain

I am trying to remove duplicate photos on my Imac   I just upgraded to elcapitain

nothing seems to work    what is the best remover you know of..

second question

I have an older Imac and want to transfer my photos and files to a newer imac   not new    but I have tried

migration assistant  and the two computer do not recognize each other ,, I have tried a firewire   that did not work

I then tried a dynex transfer cable     and the two computers will not synch... could use some suggestions

on both..... I am familiar with pc s  much simpler