SQL Server - How to insert a record and make sure it is unique

SELECT WordID FROM Words WHERE Word = @Word
IF WordID IS NULL OR WordID <= 0
    INSERT INTO Words VALUES (@Word)

INSERT INTO Words (Word)
SELECT @Word
WHERE NOT EXISTS (SELECT WordID FROM Words WHERE Word = @Word)

INSERT INTO Words (Word)
    SELECT @Word
WHERE NOT EXISTS (SELECT WordID FROM Words WHERE Word = @Word)

INSERT INTO Words (Word)
    VALUES(@Word)
WHERE NOT EXISTS (SELECT * FROM Words WHERE Word = @Word)

CREATE UNIQUE [ CLUSTERED | NONCLUSTERED ] INDEX <index_name>
    ON Words ( word [ ASC | DESC ])

ALTER TABLE Words
ADD CONSTRAINT UniqueWord
UNIQUE (Word);

insert into Words
( selectWord , Fixword)
SELECT word,'theFixword'
FROM   OldWordsTable
WHERE
(
    (word LIKE 'junk%') OR
     (word LIKE 'orSomthing')

)
and word not in
    (
    	SELECT selectWord FROM words WHERE selectWord = word
    )

CREATE UNIQUE NONCLUSTERED INDEX [IndexTableUniqueRows] ON [dbo].[table]
(
    [Col1] ASC,
    [Col2] ASC,

)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = ON, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]

declare @Error int

begin transaction
  INSERT INTO Words (Word) values(@word)
  set @Error = @@ERROR
  if @Error <> 0 --if error is raised
  begin
      goto LogError
  end
commit transaction
goto ProcEnd

LogError:
rollback transaction