I am planning to put validation logic in business logic layer which could include things like:
[Required], [Length > 0], etc. Using data annotations. However, I also need a validation rule that checks that the object is not a duplicate before having the DAL insert into the database, e.g. [IsDuplicate]. So, question is, where to put [IsDuplicate] validation rule? If I put it in my BL, then this will violate my current 3-tier setup where the BL has no knowledge of the DAL. I guess the question really becomes, is checking for duplicates considered a validation rule or something else?
Your question is ambiguous. It depends on what kind of record and what kind of operation do you refer.
If you refer to one to many record, like:
header --> many details
BL
Then the duplication check is being done in BL. That is, example, validate if the detail of a header must not contain two or more same item code, etc. And if the process is accepting array of headers, the duplicated header validation is also done in BL.
Other validation rules like minimum length, string format, null values, etc also done in BL. It can be automatically re-validated in DB though if you using some constraints and data length / isnull data type.
DAL
However, when you want to validate if the header id already exists in DB, do it in DAL. That is because BL don't know what it is in repository. It is DAL's responsibility.
There is some cases where you don't need to do validation first though, example if the header table already has unique index, it will throw exception and you just need to catch it. However for specific DB validation check like: item does not exists, item amount is not enough, specific user does not exists, you must do it in DAL, or use stored procedure for it.
But, any validation in DAL, must be called from BL and avoid direct call from UI.