SQL - Prevent foreign key field form adding duplicate values

I am creating a table and I don't want duplicates in the id_user field.  I am using below SQL statements but I can still add duplicate id_users.

CREATE TABLE SYS_PromisedDateNotif (
	pdf_id	INT	NOT NULL IDENTITY(1,1) PRIMARY KEY ,
	id_user	INT NOT NULL,
	CONSTRAINT FK_id_user FOREIGN KEY (id_user) REFERENCES BASE_User(id_user),
)

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CipherISAsked:
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Jan LouwerensSoftware EngineerCommented:
Add a unique constraint to that field.

CREATE TABLE SYS_PromisedDateNotif (
   pdf_id	INT	NOT NULL IDENTITY(1,1) PRIMARY KEY ,
   id_user	INT NOT NULL,
   CONSTRAINT FK_id_user FOREIGN KEY (id_user) REFERENCES BASE_User(id_user),
   CONSTRAINT UC_id_user UNIQUE (id_user)
)

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Jan LouwerensSoftware EngineerCommented:
Or, without giving the unique constraint a name

CREATE TABLE SYS_PromisedDateNotif (
   pdf_id	INT	NOT NULL IDENTITY(1,1) PRIMARY KEY ,
   id_user	INT NOT NULL UNIQUE,
   CONSTRAINT FK_id_user FOREIGN KEY (id_user) REFERENCES BASE_User(id_user)
)

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Scott PletcherSenior DBACommented:
You could also just get rid of the unneeded identity column and uniquely cluster the table on the id_user:

CREATE TABLE SYS_PromisedDateNotif (
      id_user      INT NOT NULL CONSTRAINT CL_id_user UNIQUE CLUSTERED,
      CONSTRAINT FK_id_user FOREIGN KEY (id_user) REFERENCES BASE_User(id_user),
)
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