PRIMARY KEY Constraint Problem

I have an odd problem on something that used to work fine.
I have an SP that inserts a record into a table (Contract) with two keyed fields.

The keys are as follows:

ContractID and SeqID (Sequence)
These two keys make the records unique.

Ex:
ContractID SeqID
12345 1
12345 2
12345 3
etc....

Several weeks of using this procedure have been fine. Suddenly I started getting this error:

Violation of PRIMARY KEY constraint 'PK_contract'. Cannot insert duplicate key in object 'Contract'.
The statement has been terminated.

I verified that the values do not violate the constraints. In fact, I can type the exact information into the table directly without a problem.

Has anybody experienced this before?

Any help would be apprciated!


Here is the code in the SP;

CREATE PROCEDURE bcipNewContractSeq @ContractID Char(10 )AS

DECLARE @MaxSeqID int
DECLARE @NewSeqID int

SELECT @MaxSeqID = Max(SeqID) from Contract_Live..Contract WHERE ContractID = @ContractID

SET @NewSeqID = @MaxSeqID + 1

--Copy Contract info for new seq with new seqid- record has default start and end dates
INSERT INTO [Contract_Live].[dbo].[Contract] ([ContractID], [seqID], [Status], [ContractName])
SELECT @ContractID, @NewSeqID, 'In Process', ContractName
FROM [Contract_Live].[dbo].[Contract]
WHERE [Contract_Live].[dbo].[Contract] .ContractID = @ContractID

GO