You should have a primary key. Probably CERT_ID is the likely. For the other column use a UNIQUE restriction:
CREATE TABLE [dbo].[FORUM_CERTIFICATES] (
[TOPIC_ID] [int] UNIQUE NOT NULL ,
[ExpDate] [nvarchar] (14) COLLATE Latin1_General_CI_AS NULL ,
[IssueDate] [nvarchar] (14) COLLATE Latin1_General_CI_AS NULL ,
[CERT_ID] [int] PRIMARY KEY NOT NULL
) ON [PRIMARY]
GO