Need help with a measure

[Shamatix]

Hello fellow users of Power Bi,

Well ill try to describe one of my issues:

=If(HASONEVALUE(Indfortoldning[Løbenummer]),AVERAGE(Indfortoldning[Moms grundlag]),SUMX(SUMMARIZE(Indfortoldning,Indfortoldning[Løbenummer],"ABCD",AVERAGE(Indfortoldning[Moms grundlag])),[ABCD]))

So the above measure does the correct, what I hadnt realized was there can be multiple different invoices with the same "løbenummer" and different "Moms grundlag" so if that is the case I want the value to be blank. Lets say we have the below scenario

Invoice 1 

Løbenummer 123

Moms grundlag 999

Invoice 2 

Løbenummer 123

Moms grundlag 123

The above 2 invoices has same løbenummer but different momsgrundlag, if that is the case I dont want it to take the average or top 1 (both should give same result) I want the value to become blank as if there isnt typed anything into the "moms grundlag" field.

My entire issue is I am doing an SQL query and 1 invoice can have multiple lineitems, so even if "Moms grundlag" is supposed to only have 1 value pr invoice it spits it out as many times as there is lineitems. Qlikview simply take either the average or the top 1 of this value (aint sure how it does it) and in qlikview it can somehow differentiate between two invoices with the same løbenummer, i guess it just makes moms grundlag blank if the same løbenummer exists multiple times but with different moms grundlag.

http://ge.tt/3RvU3kn2

I have uploaded some sample data here and hopefully described it well enough, otherwise feel free to text me back 
I also do have skype: Snaske2 if that will make the communication easier

hope someone can help..

Best regards