Arturo, 2828 22 bytes
$=>[=squeeze<=sort squeeze&]$=>[=unique<=squeeze&] $=>[ ; a function where input is assigned to & squeeze& ; squeeze the input; e.g. [1 1 2 1 1 1 3 3] -> [1 2 1 3] sort ; sort <= ; duplicate squeezeunique ; squeezeremove duplicates = ; are they equal? ] ; end function