hi, can help me to get the cell value from different sheets without duplicate.

my table as below,

sheet 1
column a, column b,
001 , nn
002 ,
003 , rr
004 ,

sheet 2
column a, column b,
001 , rr
002 ,
003 ,
004 , nn

sheet 3
column a, column b,
001 , nn
002 , nn
003 ,
004 ,


result i want is..

sheet 4
column a, column b,
001 , nn, rr (no duplicate, only one 'nn')
002 , nn
003 , rr
004 , nn


the formula that i use in (sheet 4,column b) is:
=TEXTJOIN(",",true,
IF(a1=sheet1!$a$1:$a$4,sheet1!$B$1:$b$4,
IF(a1=sheet2!$a$1:$a$4,sheet2!$B$1:$b$4,
IF(a1=sheet3!$a$1:$a$4,sheet3!$B$1:$b$4,
""))))

the answer that i got from this formula (column b1) is 'nn,rr,nn'.
how do i include (isnumber) or (iferror) or (countif) or anything to remove duplicate value in cell, so that my answer is 'nn,rr'

tq tq