Spreadsheet limits

Hi people

I have trouble getting a complex formula to work properly. I'm trying to extract unique values from a large list of city names. The list is R986:C1 long and has empty cells in between. I found this array function on the web:

{=IFERROR(INDEX(Kupong_Reg!$D$2:$D$986; MATCH(0; FREQUENCY(IF(EXACT(Kupong_Reg!$D$2:$D$986; TRANSPOSE($C$1:C1)); MATCH(ROW(Kupong_Reg!$D$2:$D$986); ROW(Kupong_Reg!$D$2:$D$986)); ""); MATCH(ROW(Kupong_Reg!$D$2:$D$986); ROW(Kupong_Reg!$D$2:$D$986))); 0)); "")}

And it worked perfectly when I tested it on a smaller range. R10:C1. 

I'm wondering if the Office 365 Excel app is choking on this one, I have put this function in 100 rows. 

The spreadsheet has additional functions. I believe there are 6000 cells with functions in them. Divided into 4 sheets.

Am I hitting any limits?

Regards

Morten