Access duplicate error

When importing a table from excel I am trying to assign a particular field as the primary key. However access is telling me that I cannot assign it as a primary key and the following text comes up:
"The changes you requested were not successful because they would create duplicate values in the index, primary key, or relationship. Change the data in the field or fields that contain duplicate data, remove the index, or redefine the index to permit duplicate entries and try again."

This is the first table in the database, and having checked the list in excel I can see that there are no duplicates in existence in the column/field and yet the error comes up. The list is approximately 15000 records long. Would there be a reason why the error message comes up?