Assume your data at A1 looks like this:
ListMa
L
C
J
I
B
F
H
D
J
B
B
J
D
H
B
J
D
C
and you want to generate this list:
ListMa
C
J
B
H
D
Create these 2 cells at B1
XYZZ
FALSE
B1 can be any text but ListMa, or leave blank.
B2 has the formula
=COUNTIF($A$2:$A$19,A2)1
Data Filter Advanced Filter Copy to another location
List Range: $A$1:$A$19
Criteria Range: $B$1:$B$2
Copy to: $C$1
Check Unique Records only
There are many ways to make mistakes here.
The safer, more flexible, updatable way is with Pivot Table.