Textjoin for multiple sheets without duplicate value

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

I think you need to update your profile. If you are using Textjoin, you are probably on 365 or 2021, right?
It's easier for us to give you an accurate answer if you supply a good sample workbook. Please read the yellow banner at the top of the page

hi
yes, i m using 365. but my colleague is using excel 2016 and excel 2019.. so basically i need to use those old version of formula, so that they can read/view as well

so you can't use TEXTJOIN. You didn't attach a sample workbook. I think this is going to be much harder without v365.