Unique key Voilation error....

"Violation of UNIQUE KEY constraint 'UniqueVisit'.
Cannot insert duplicate key in object 'dbo.Visit\r\nThe statement has been terminated.
\r\nThe 'Visit_Insert'
procedure attempted to return a status of NULL, which is not allow...

CREATE TABLE [dbo].[Visit](
	[ID] [bigint] IDENTITY(1,1) NOT NULL,
	[VerId] [bigint] NULL,
	[ClientId] [bigint] NOT NULL,
	[VisitDate] [datetime] NOT NULL,
	[VisitId] [char](10) NULL,
	[Child] [nvarchar](50) NULL,
	[CurrentMeth] [nvarchar](50) NULL,
	[FollUp] [nvarchar](50) NULL,
	[GestAge] [nvarchar](50) NULL,
	[MedicalRef] [nvarchar](50) NULL,
	[Sensitive] [nvarchar](50) NULL,
	[MarketingRef] [nvarchar](50) NULL,
	[Category] [nvarchar](50) NULL,
 CONSTRAINT [aaclientTransaction_PK] PRIMARY KEY NONCLUSTERED 
(
	[ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY],
 CONSTRAINT [UniqueVisit] UNIQUE NONCLUSTERED 
(
	[ClientId] ASC,
	[VisitId] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO
SET ANSI_PADDING OFF
GO
ALTER TABLE [dbo].[Visit]  WITH NOCHECK ADD  CONSTRAINT [FK_clientTransactions_Client] FOREIGN KEY([ClientId])
REFERENCES [dbo].[Client] ([ID])
ON UPDATE CASCADE
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[Visit] NOCHECK CONSTRAINT [FK_clientTransactions_Client]
GO
ALTER TABLE [dbo].[Visit]  WITH NOCHECK ADD  CONSTRAINT [FK_ServicePointStatistic_Version] FOREIGN KEY([VersionId])
REFERENCES [dbo].[Version] ([ID])
ON UPDATE CASCADE
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[Visit] CHECK CONSTRAINT [FK_ServicePointStatistics_Version]

ALTER Procedure [dbo].[Visit_Insert]
	@VerId bigint, 
	@ClientId bigint, 
	@VisitDate datetime, 
	@VisitId char(10),
	@Child nvarchar(50),
	@CurrentMeth nvarchar(50),
	@FollUp nvarchar(50),
	@GestAge nvarchar(50),
	@MedicalRef nvarchar(50),
	@Sensitive nvarchar(50),
	@MarketingRef nvarchar(50),
	@Category nvarchar(50),
	@Id bigint OUTPUT
	
AS

insert into Visit  
					(VerId,
					ClientId,
					VisitDate,
					VisitId,
					Child,
					CurrentMeth,
					FollUp,
					GestAge,
					MedicalRef,
					Sensitive,
					MarketingRef,
					Category)
					values
					(@VerId,
					@ClientId,
					@VisitDate,
					@VisitId,
					@Child,
					@CurrentMeth,
					@FollUp,
					@GestAge,
					@MedicalRef,
					@Sensitive,
					@MarketingRef,
					@Category)
SET @Id = @@IDENTITY

RETURN @Id