When I import an SD File and choose to merge it with an existing database with the option to skip the record if a similar record already exists, I find that the data is still importing, when it shouldn't be.
If the field you are using to do the comparison has a field name that is two words, the Access SQL processor will reject the query. For example, if the field name you are using to do the comparison is "ID NUMBER", the merge will not work properly.
To get around this, you can choose "custom SQL" for the match criterion in the Merge tab of the Import dialog box, and enter the following in the "SQL For Import Match" dialog:
[ID NUMBER] = '$ID NUMBER'
The other way you can get around it is to rename the field to a single word, like ID_NUMBER. You could do this in the database using MS Access, and in the SDFile using Notepad or Word.
A third possibility is to open the database in MS Access and set ID Number as the Primary Key field.