Avoid or filter duplicated function signature entries

[radare]

Way to reproduce:

$ r2 /bin/ls
[0x100003a84]> aaa
[0x100003a84]> zg
[0x100003a84]> zg
[0x100003a84]> zg
[0x100003a84]> z* $$~N
za main_3 N sym.func.1000047d0_3
za main_2 N sym.func.1000047d0_2
za main N sym.func.1000047d0
[0x100003a84]>