Getting constraint errorr for INSERT? Cannot insert duplicate key row in object 'etag.Tag_Processed' with unique index 'IX_Tag_Processed'

Question

I have an index constraint "IX_Tag_Processed" on the field "Tag_Name" for the table "Tag_Processed". I keep getting this constraint error:
Msg 2601, Level 14, State 1, Line 15
Cannot insert duplicate key row in object 'etag.Tag_Processed' with unique index 'IX_Tag_Processed'. The duplicate key value is (AZPS_TEMUWS0110BL4_CISO).
The statement has been terminated.
For this INSERT: I have tried using tagstg.Tag_Name NOT IN with same result:
INSERT into [Forecast_Data_Repository].[etag].[Tag_Processed] (Tag_Name, Tag_Type,Start_Datetime, End_Datetime, Source_SC, Sink_SC, Source_CA, Sink_CA, Source, Sink, Load_dt, Energy_product_code_id)
SELECT DISTINCT (Tag_Name), Tag_Type,Start_Datetime, End_Datetime, Source_SC, Sink_SC, Source_CA, Sink_CA, Source, Sink, GETUTCDATE(),  [Forecast_Data_Repository].rscalc.GetStubbedEngProductCodeFromStaging(tagstg.Tag_Name)
FROM [Forecast_Data_Repository].[etag].[Tag_Stg] tagstg
WHERE tagstg.Id BETWEEN @minTId AND @maxTId --AND
--tagstg.Tag_Name NOT IN (
-- SELECT DISTINCT tproc.Tag_Name from [Forecast_Data_Repository].[etag].[Tag_Processed] tproc
thank you  in advance, 
Greg Hanson

Answer 1

I have even tried a merge with the same constraint error,
DECLARE @minTId bigint, @minTRId bigint, @minEId bigint
DECLARE @maxTId bigint, @maxTRId bigint, @maxEId bigint
DECLARE @errorCode int
DECLARE @ReturnCodeTypeIdName nvarchar(50)
SELECT @minTRId = Min(Id) FROM [etag].[Transmission_Stg]
SELECT @maxTRId = Max(Id) FROM [etag].[Transmission_Stg]
SELECT @minTId = Min(Id) FROM [etag].[Tag_Stg]
SELECT @maxTId = Max(Id) FROM [etag].[Tag_Stg]
DECLARE @MergeOutputTag TABLE
ActionType NVARCHAR(10),
InsertTagName NVARCHAR(50)
--UpdateTagName NVARCHAR(50)
--DeleteTagName NVARCHAR(50)
DECLARE @MergeOutputEnergy TABLE
ActionType NVARCHAR(10),
InsertTagId BIGINT
--UpdateTagName NVARCHAR(50)
--DeleteTagName NVARCHAR(50)
DECLARE @MergeOutputTransmission TABLE
ActionType NVARCHAR(10),
InsertTagId BIGINT
--UpdateTagName NVARCHAR(50)
--DeleteTagName NVARCHAR(50)
MERGE [Forecast_Data_Repository].[etag].[Tag_Processed] tagProc
USING [Forecast_Data_Repository].[etag].[Tag_Stg] tagStg
ON 
tagProc.Tag_Name = tagStg.Tag_Name AND
tagProc.Tag_Type = tagStg.Tag_Type AND
tagProc.Start_Datetime = tagStg.Start_Datetime AND
tagProc.End_Datetime = tagStg.End_Datetime AND
tagProc.Source_SC = tagStg.Source_SC AND
tagProc.Source_CA = tagStg.Source_CA AND
tagProc.Sink_CA = tagStg.Sink_CA AND
tagProc.Source = tagStg.Source AND
tagProc.Sink = tagStg.Sink 
WHEN MATCHED THEN
UPDATE
SET Tag_Name = tagStg.Tag_Name,
Tag_Type = tagStg.Tag_Type,
Start_DateTime = tagStg.Start_Datetime,
End_Datetime = tagStg.End_Datetime,
Source_SC = tagStg.Source_SC,
Sink_SC = tagStg.Sink_SC,
Source_CA = tagStg.Source_CA,
Sink_CA = tagStg.Sink_CA,
Source = tagStg.Source,
Sink = tagStg.Sink,
Load_dt = GETUTCDATE()
WHEN NOT MATCHED BY TARGET THEN
INSERT (Tag_Name, Tag_Type, Start_Datetime, End_Datetime, Source_SC, Sink_SC, Source_CA, Sink_CA, Source, Sink, Load_dt)
VALUES (tagStg.Tag_Name, tagStg.Tag_Type, tagStg.Start_Datetime, tagStg.End_Datetime, tagStg.Source_SC, tagStg.Sink_SC, tagStg.Source_CA, tagStg.Sink_CA, tagStg.Source, tagStg.Sink, GETUTCDATE())
OUTPUT
$action,
INSERTED.Tag_Name
--UPDATED.Tag_Name
INTO @MergeOutputTag;
SELECT * FROM @MergeOutputTag;
Greg Hanson