problems with updating references

[caboom]

Hello, I am hoping someone can help me with a error I get when I try to update references.

I am working on a project where I use architect drawings as a background drawings. I draw my installations on top of these drawings.

I have made shuttle dwgs with the drawings I receive and set them up as referenced viewports in my drawings. When I get an update from the architect, I make new shuttle drawings with the same name and simply update these references. 8 out 10 times it all goes very smoothly, but occasionally I get a message:"Duplicate reference encountered. Operation canceled.....The Class Definition named "NNA#7_0" cannot be referenced because a Format with that name already exists in the current document." Sometimes, I can just rename the class, but this time I can't find the definition "NNA#7_0" anywhere in either of the drawings, not as a class not as a symbol or hatch...so I can't seem to solve the problem. I've tried purging or cutting and pasting in a new drawing but the error just stays.

Thanks to anyone who takes the time to read and answer!