I've purchased a west highland terrier and applied for the 5 generation pedigree certificate. I noticed in the  great great granparents line the same stud dogs name appears in both  my dogs parents blood line, is this acceptable in breeding practice. At what point is it interbreeding?

I think guest you mean inbreeding.  This and the term linebreeding are sownmwhat related terms which mean mating of related individuals and generally the difference is the degree.

To some extend mating two dogs of the smae breed will be mating related animals togerher as of course they are the same breed so will be more rlated than they woudl be to one of a different breed, and even then some breeds are related.  so ti is all a case iof degree.

If you think about it the fact that your dog has the smae dog twice in the fourth generation to many people woudl barely be linebreding.

There are 16 ancestors in the 4th generation making each 6.25% of the pups bloodline, so twice just makes it the eqyuvakent to a great grandparent.

In fact first cousins in human terms can marry that would put the same two individuals twice in the third generation of any chikld of such a marriage, in other words they woudl only have 6 great grandparents instead of 8.