Error when inserting data in non-unique index - sql-server

Question

I have a batch job that synchronizes data between two identical databases using the MERGE statement. Sometimes, I get the following error: cannot insert duplicate key row in object 'dbo.MatchPlayerStatistics' with unique index 'IX_MatchPlayerStatistics_Player'. That does not make sense because IX_MatchPlayerStatistics_Player is not a unique index, and I am able to insert the exact same data manually. If I drop the index, insert the data, then recreate the index, it works fine.
Why does this error occur and how can I prevent it?
More information
The creation script for the table is the following:
CREATE TABLE [dbo].[MatchPlayerStatistics](
[ExternalID] [nvarchar](50) NULL,
[ProviderID] [int] NOT NULL,
[MatchID] [int] NOT NULL,
[PlayerPersonID] [int] NOT NULL,
[TeamID] [int] NOT NULL,
[YellowCards] [smallint] NULL,
[DoubleYellowCards] [smallint] NULL,
[RedCards] [smallint] NULL,
[Fouls] [smallint] NULL,
[Goals] [smallint] NULL,
[PenaltyGoals] [smallint] NULL,
[PenaltiesMissed] [smallint] NULL,
[PenaltiesSaved] [smallint] NULL,
[Shots] [smallint] NULL,
[Attacks] [smallint] NULL,
[Corners] [smallint] NULL,
[Offsides] [smallint] NULL,
[Assists] [smallint] NULL,
[OwnGoals] [smallint] NULL,
[GoalsConcedeed] [smallint] NULL,
[CreatedBy] [int] NOT NULL,
[CreatedOn] [datetime] NOT NULL,
[ModifiedBy] [int] NOT NULL,
[ModifiedOn] [datetime] NOT NULL,
CONSTRAINT [PK_MatchPlayerStatistics] PRIMARY KEY CLUSTERED
(
[ProviderID] ASC,
[MatchID] ASC,
[PlayerPersonID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
CREATE NONCLUSTERED INDEX [IX_MatchPlayerStatistics_Player] ON [dbo].[MatchPlayerStatistics]
(
[ProviderID] ASC,
[PlayerPersonID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
The MERGE statement is the following:
MERGE MatchPlayerStatistics AS M
USING (
SELECT [ExternalID]
,[ProviderID]
,[MatchID]
,[PlayerPersonID]
,[TeamID]
,[YellowCards]
,[DoubleYellowCards]
,[RedCards]
,[Fouls]
,[Goals]
,[PenaltyGoals]
,[PenaltiesMissed]
,[PenaltiesSaved]
,[Shots]
,[Attacks]
,[Corners]
,[Offsides]
,[Assists]
,[OwnGoals]
,[GoalsConcedeed]
,[CreatedBy]
,[CreatedOn]
,[ModifiedBy]
,[ModifiedOn]
FROM [Replication].MatchPlayerStatistics
WHERE ProviderID = 1
) AS R
ON (M.MatchID = R.MatchID AND M.PlayerPersonID = R.PlayerPersonID AND M.ProviderID = R.ProviderID )
WHEN NOT MATCHED
THEN
INSERT ([ExternalID]
,[ProviderID]
,[MatchID]
,[PlayerPersonID]
,[TeamID]
,[YellowCards]
,[DoubleYellowCards]
,[RedCards]
,[Fouls]
,[Goals]
,[PenaltyGoals]
,[PenaltiesMissed]
,[PenaltiesSaved]
,[Shots]
,[Attacks]
,[Corners]
,[Offsides]
,[Assists]
,[OwnGoals]
,[GoalsConcedeed]
,[CreatedBy]
,[CreatedOn]
,[ModifiedBy]
,[ModifiedOn])
VALUES
( R.[ExternalID]
,R.[ProviderID]
,R.[MatchID]
,R.[PlayerPersonID]
,R.[TeamID]
,R.[YellowCards]
,R.[DoubleYellowCards]
,R.[RedCards]
,R.[Fouls]
,R.[Goals]
,R.[PenaltyGoals]
,R.[PenaltiesMissed]
,R.[PenaltiesSaved]
,R.[Shots]
,R.[Attacks]
,R.[Corners]
,R.[Offsides]
,R.[Assists]
,R.[OwnGoals]
,R.[GoalsConcedeed]
,R.[CreatedBy]
,R.[CreatedOn]
,R.[ModifiedBy]
,R.[ModifiedOn])
WHEN MATCHED
THEN
UPDATE
SET [ExternalID] = R.[ExternalID]
,[ProviderID] = R.[ProviderID]
,[MatchID] = R.[MatchID]
,[PlayerPersonID] = R.[PlayerPersonID]
,[TeamID] = R.[TeamID]
,[YellowCards] = R.[YellowCards]
,[DoubleYellowCards] = R.[DoubleYellowCards]
,[RedCards] = R.[RedCards]
,[Fouls] = R.[Fouls]
,[Goals] = R.[Goals]
,[PenaltyGoals] = R.[PenaltyGoals]
,[PenaltiesMissed] = R.[PenaltiesMissed]
,[PenaltiesSaved] = R.[PenaltiesSaved]
,[Shots] = R.[Shots]
,[Attacks] = R.[Attacks]
,[Corners] = R.[Corners]
,[Offsides] = R.[Offsides]
,[Assists] = R.[Assists]
,[OwnGoals] = R.[OwnGoals]
,[GoalsConcedeed] = R.[GoalsConcedeed]
,[CreatedBy] = R.[CreatedBy]
,[CreatedOn] = R.[CreatedOn]
,[ModifiedBy] = R.[ModifiedBy]
,[ModifiedOn] = R.[ModifiedOn]
WHEN NOT MATCHED BY SOURCE AND M.ProviderID = 1
THEN DELETE;
The [Replication].MatchPlayerStatistics table is an intermediate table that is filled with data from the [dbo].MatchPlayerStatistics table on another copy of the database. The schema for all these tables is the same.

Answer 1

One idea I have is to check and see if there are triggers on the table and if one of them is inserting to another table that has a unique index.
Looking at your code, here is a completely wild guess. You are using IGNORE_DUP_KEY = OFF which is normally only used for unique constraints, so I wonder if it is considering it a unique constraint even without the unique keyword. Try creating the index without that phrase and see what happens.