Cannot insert duplicate key row in object 'dbo.CFD' with unique index - sql-server

Question

I am running into an error on the insertion of a single row into a table, dbo.CFD. The complete error states:
Cannot insert duplicate key row in object 'dbo.CFD' with unique index 'IX_CFD_NRSC_FXTR_N'. The duplicate key value is (BP2001, 1990, CHK, 01, 3456).
Looking at the index in question, IX_CFD_NRSC_FXTR_N, it is a non-unique, non-clustered index on a single column (The column with BP2001 in the error).
Looking at the data, there are many records with the values of BP2001, CHK, 01, and 3456, however none with 1990 (but various other values), so it doesn't seem I am conflicting with a unique constraint.
I also verified that all foreign key constraints are being met and the primary key constraint is not being violated.
How can I figure out what exactly is causing this issue? Every example I can find results in there being duplicate data being inserted, but I don't think that is the issue here. Thanks!
Edit----------
The table create script:
CREATE TABLE [dbo].[CFD](
[PRPSL_I] [int] NOT NULL,
[FILE_TYPE_C] [char](3) NOT NULL,
[CAD_FLR_C] [char](2) NOT NULL,
[FXTR_SERL_I] [int] NOT NULL,
[ADJC_I] [int] NULL,
[CMP_I] [int] NULL,
[VER_I] [int] NULL,
[NRSC_FXTR_N] [varchar](15) NULL,
[MDSE_SPCE_HT_I] [int] NULL,
[MDSE_SPCE_LGTH_I] [int] NULL,
[MDSE_SPCE_DPT_I] [int] NULL,
[NRSC_FXTR_SIDE_I] [int] NULL,
[NRSC_FXTR_SECT_I] [int] NULL,
[BOFM_INCL_F] [bit] NOT NULL,
[DIVI_I] [int] NULL,
[PRTY_I] [int] NULL,
[REUSE_I] [int] NULL,
[FXTR_ISTL_LOC_N] [varchar](6) NULL,
[PHYS_SECT_NUM_I] [int] NULL,
[SAFL_LOC_N] [varchar](4) NULL,
[SEQ_I] [int] NULL,
[SIDE_ASLE_I] [int] NULL,
[LDIN_I] [int] NULL,
[LDIN_STAT_I] [int] NULL,
CONSTRAINT [PK_CFD] PRIMARY KEY CLUSTERED
(
[PRPSL_I] ASC,
[FILE_TYPE_C] ASC,
[CAD_FLR_C] ASC,
[FXTR_SERL_I] ASC
)WITH (PAD_INDEX = ON, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO
ALTER TABLE [dbo].[CFD] ADD CONSTRAINT [DF_CFD_BOFM_INCL_F] DEFAULT ((0)) FOR [BOFM_INCL_F]
GO
ALTER TABLE [dbo].[CFD] WITH CHECK ADD CONSTRAINT [FK1_CFD] FOREIGN KEY([PRPSL_I], [FILE_TYPE_C], [CAD_FLR_C])
REFERENCES [dbo].[CAD_FILE] ([PRPSL_I], [FILE_TYPE_C], [CAD_FLR_C])
GO
ALTER TABLE [dbo].[CFD] CHECK CONSTRAINT [FK1_CFD]
GO
ALTER TABLE [dbo].[CFD] WITH CHECK ADD CONSTRAINT [FK2_CFD] FOREIGN KEY([CMP_I])
REFERENCES [dbo].[CMP] ([CMP_I])
GO
ALTER TABLE [dbo].[CFD] CHECK CONSTRAINT [FK2_CFD]
GO
ALTER TABLE [dbo].[CFD] WITH CHECK ADD CONSTRAINT [FK3_CFD] FOREIGN KEY([ADJC_I])
REFERENCES [dbo].[ADJC_E] ([ADJC_I])
GO
ALTER TABLE [dbo].[CFD] CHECK CONSTRAINT [FK3_CFD]
GO
ALTER TABLE [dbo].[CFD] WITH CHECK ADD CONSTRAINT [CK_CFD] CHECK (([FILE_TYPE_C]<>'SFP'))
GO
ALTER TABLE [dbo].[CFD] CHECK CONSTRAINT [CK_CFD]
GO
CREATE NONCLUSTERED INDEX [IX_CFD_NRSC_FXTR_N] ON [dbo].[CFD]
(
[NRSC_FXTR_N] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO