Javascript: Find douplicated values from array with keys

Title is pretty much self explanatory...

I want to be able to find duplicated values from JavaScript array.

The array keys can be duplicated so I need to validate only the array values.

Here is an example :

var arr=[
    Ibanez:     'JoeSatriani',
    Ibanez:     'SteveVai',
    Fender:     'YngwieMalmsteen',
    Fender:     'EricJohnson',
    Gibson:     'EricJohnson',
    Takamine:   'SteveVai'
];

In that example:

the key is the guitar brand the value is the guitar player name.

So:

If there is duplicated keys (like: Ibanez or Fender) as on that current example that is OK :-)

But

If there is duplicated values (like: EricJohnson or SteveVai) I'm expecting to get (return) that error:

EricJohnson,SteveVai