AWK Find Duplicate Value in Column 3. Print Entire Line

Question

Why doesn't this work? I've looked for so long and have found some pretty complex solutions, but I'm thinking this can be simplified and reused...sad :'(

Statement

awk -F"\t" '!seen[$3]++'

File

r1c1    r1c2    r1c3
r2c1    r2c2    r2c3
r3c1    r3c2    r3c3
r4c1    r4c2    r3c3
r5c1    r5c2    r5c3

Desired Output

r3c1    r3c2    r3c3
r4c1    r4c2    r3c3

Code adds a 0 and 1.

[user@host]$ awk '{a[$3]=a[$3] $0 RS c[$3]++} END {for (i in c) if (c[i]>1) printf "%s",a[i]}' file
r3c1    r3c2    r3c3
0r4c1   r4c2    r3c3
1[jcole@dukescri01 srlg]$ 

Answer 1

In awk, one-pass version that stores records to hash:

$ awk '
{
    a[$3]=a[$3] $0 RS        # store records
    c[$3]++                  # counter
}
END {
    for(i in c)
        if(c[i]>1)           # pick the ones with duplicates
            printf "%s",a[i]
}' file
r3c1    r3c2    r3c3
r4c1    r4c2    r3c3