Sql-server – Simple insert in stored procedure runs twice

I have one stored procedure to insert into a table. I have two questions:

1. Is it a ok?

2. Sometimes the insert operation runs twice. Why?

Microsoft SQL Server 2016 (RTM) – 13.0.1601.5 (X64)
  Apr 29 2016 23:23:58
  Copyright (c) Microsoft Corporation
  Enterprise Edition (64-bit) on Windows 8.1 Enterprise 6.3 (Build 9600: )

CREATE PROCEDURE [dbo].[insert_newNEWS] 
-- Add the parameters for the stored procedure here
@news_owner_id   INT,
@news_owner_type BIT,
@news_cat_id     INT,
@news_producer   NVARCHAR(1000),
@news_title      NVARCHAR(1000),
@news_sutitr     NVARCHAR(1000),
@news_text       NVARCHAR(1000),
@news_date       NVARCHAR(128),
@news_time       NVARCHAR(16),
@news_link       NVARCHAR(512),
@img_uri_type    TINYINT,
@img_uri         XML,
@newsContentID   INT OUTPUT
AS
     BEGIN


         -- SET NOCOUNT ON added to prevent extra result sets from
     -- interfering with SELECT statements.
     SET NOCOUNT ON;
     BEGIN TRANSACTION;
     SET @newsContentID = dbo.Biggest_ID(13);
     INSERT INTO [Contents].[news]
     ([id_news],
      [news_owner_id],
      [news_owner_type],
      [news_cat_id],
      [news_producer],
      [news_title],
      [news_sutitr],
      [news_text],
      [news_date],
      [news_time],
      [news_link],
      [img_uri_type],
      [img_uri]
     )
     VALUES
     (@newsContentID,
      @news_owner_id,
      @news_owner_type,
      @news_cat_id,
      @news_producer,
      @news_title,
      @news_sutitr,
      @news_text,
      @news_date,
      @news_time,
      @news_link,
      @img_uri_type,
      @img_uri
     );
     IF(@@ERROR <> 0)
         BEGIN
             ROLLBACK TRAN;
             RETURN 1;
         END;
     COMMIT;
 END;

AND this is Mr Biggest_ID function

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER FUNCTION [dbo].[Biggest_ID](@table_id INT)
RETURNS INT
AS
     BEGIN
         DECLARE @BiggestID AS INT;
         --another table ID
         IF @table_id = 13
             BEGIN
                 SELECT @BiggestID = ISNULL(MAX(nws.id_news), 0) + 1
                 FROM [Contents].[news] AS nws;
             END;

         RETURN @BiggestID;
     END;

and this table :

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE TABLE [Contents].[news](
     [id_news] [int] NOT NULL,
     [news_owner_id] [int] NOT NULL,
     [news_owner_type] [bit] NOT NULL,
[news_cat_id] [int] NOT NULL,
[news_producer] [dbo].[JBText] NOT NULL,
[news_title] [dbo].[JBText] NOT NULL,
[news_sutitr] [dbo].[JBText] NOT NULL,
[news_text] [dbo].[JBText] NOT NULL,
[news_date] [nvarchar](128) NULL,
[news_time] [nvarchar](16) NULL,
[news_link] [dbo].[JBLinks] NULL,
[img_uri_type] [tinyint] NULL,
[img_uri] [xml] NULL,
 CONSTRAINT [PK_news] PRIMARY KEY CLUSTERED 
(
    [id_news] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

GO