I have a file like this:

[token1]=value1
[token2]=value2
.
.
.
[token n]=valuen

uniq -cd prueba1.txt

$ cat prueba1.txt
uno
dos
tres
tres
cuatro
cinco
seis
cuatro
siete
ocho
$ uniq -cd prueba1.txt
      2 tres

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---------- Post updated at 05:09 PM ---------- Previous update was at 05:09 PM ----------

Sort your file first - 'man sort'

Why don't you sort before running uniq?

Also, just a starting point, look at arrays usage in awk:

 
awk '
{
        key = $0; # use your key field here
        if(key in regarr) {
                duparr[key] = key
        }
        else {
                regarr[key] = key
        }
}
END {
        for(idx in duparr) {
                print idx;
        }
}
'

Thanks guys
I appreciate your flash responses, this worked better than I expected
My final version is this command:

tr ' ] ' ' ' < prueba.txt | sort | awk '{print $1} ' | uniq -d | wc -l

A little bit smaller:

 cut -d']' -f1 prueba2.txt|sort|uniq -d|wc -l

Nice one, at least is smaller that mine, hehe