LINQ to SQL: Insert non-identity Primary Key

I am using LINQ to SQL for the first time on a small project. My tables have Primary Keys that are not identity columns because I am essentially importing from another database. The relationship is a many-to-many with a linking table. Is there a way to tell LINQ to ignore the insert if it is a duplicate?

I have already tried to check the current tables if the object exists and if so, skip to the next but because of the relationships, it makes it difficult.

I can give more details if necessary.

From stackoverflow

• Instead of querying for the whole object what you can do is an Any query...

  DB.Items.Any(i => i.ID == myObj.ID);
  

  This will do an optimized query for checking if it exists first.

  As far as having LINQ to SQL handle this for you, I'm pretty sure it cannot.