Preventing Duplicate Rows On Insert

I have a table using an identity column as its Primary Key and two
columns (table reduced for simplicity) EmployeeNumber and ArrivalTime.

CREATE TABLE [tblRecords] (
[ID] [bigint] IDENTITY (1, 1) NOT NULL ,
[EmployeeNumber] [varchar] (10) COLLATE SQL_Latin1_General_CP1_CI_AS
NOT NULL ,
[ArrivalTime] [datetime] NOT NULL ,
CONSTRAINT [PK_tblRecords] PRIMARY KEY CLUSTERED
(
[ID]
) ON [PRIMARY]
) ON [PRIMARY]
GO

I have an insert procedure that checks for duplicates before inserting
a new record:

IF (SELECT TOP 1 [ID] FROM tblRecords WHERE EmployeeNumber =
@SocialSecurity) IS NULL
BEGIN
INSERT INTO tblRecords(EmployeeNumber,ArrivalTime)
VALUES (@EmployeeNumber, @ArrivalTime)
SELECT SCOPE_IDENTITY()
END
ELSE
SELECT 0 AS DuplicateRecord

In 99.9% of the cases, this works well. However, in the event that the
insert attempts are literally "ticks" apart, the "SELECT TOP 1..."
command completes on both attempts before the first attempt completes.
So I end up with duplicate entries if the procedure is called multiple
times vey quickly. The system needs to prevent duplicate
EmployeeNumbers within the past 45 days so setting the EmployeeNumber
to UNIQUE would not work. I can check for older entries (45 days or
newer) very easily, but I do not know how to handle the times when the
procedure is called multiple times within milliseconds. Would a
TRANSACTION with a duplicate check after the INSERT with a ROLLBACK
work in this case? Any help is greatly appreciated!

-E