IF(ISERROR Array formula

[Tablecloth98]

I'm trying to write a formula that looks for duplicates and returns a specific text value if a duplicate is present. I'd ideally like to do this without nesting multiple if formulas as this might slow down the workbook.
What I have so far is:
=IF(ISERROR(MATCH(Sheet1!$I3,Sheet2!$I:$I&Sheet3!$I:$I,0)),"","Dup")
It doesn't seem to be picking up duplicates at all.
Where am I going wrong?

 

[Scott Huish]

=IF(COUNTIF(Sheet2!I:I,Sheet1!I3)+COUNTIF(Sheet3!I:I,Sheet1!I3)>1,"Dup","")

This is assuming that the value has to be on both sheets 2 and 3 to count as a duplicate. If it only has to be on one of the other sheets change the >1 to >0

 

[Tablecloth98]

=IF(COUNTIF(Sheet2!I:I,Sheet1!I3)+COUNTIF(Sheet3!I:I,Sheet1!I3)>1,"Dup","")

This is assuming that the value has to be on both sheets 2 and 3 to count as a duplicate. If it only has to be on one of the other sheets change the >1 to >0

Sorry for the late reply (It's a work thing so I'm on a funny schedule). It's coming up as #Value for some reason when I input some duplicate data

 

[Scott Huish]

I didn't match your absolute references
=IF(COUNTIF(Sheet2!$I:$I,Sheet1!$I3)+COUNTIF(Sheet3!$I:$I,Sheet1!$I3)>1,"Dup","")

but it works fine for me.

 

[Tablecloth98]

I didn't match your absolute references
=IF(COUNTIF(Sheet2!$I:$I,Sheet1!$I3)+COUNTIF(Sheet3!$I:$I,Sheet1!$I3)>1,"Dup","")

but it works fine for me.

You're right. I messed up the format a bit thank you for your help! You're the best