st_intersection is duplicating rows

Question

I'm using the sf package in R to understand how land cover changed from forest fires in BC, by overlapping burned areas (NBAC) with data from Dynamic World (DW). I have the DW data in the form of 5km by 5km grids with land cover proportions for each grid. When I use st_intersect(sf_2018, burn_2018), it duplicated four of the rows, with different geometries. Using st_intersection(sf_2018, st_difference(burn_2018)) does not resolve the problem, as suggested in this question. How can I avoid this?

Here is my code:

#Open Dynamic World Data
dw_2018 = read.csv("landcover_2018.csv")

##We can make this into a spatial object using the geoJSON information stored in .geo
sf_2018 = st_as_sf(data.frame(dw_2018, geom=geojson_sf(dw_2018$.geo)))

#Remove .geo column
sf_2018 = subset(sf_2018, select = -c(.geo))

#Open 2018 burned area data.
burn_2018 = st_read("nbac_2018.shp")

#Put a Thompson-Nicola Regional District (TNRD) shapefile into same CRS as burned areas
TNRD_conal = st_transform(TNRD, crs = crs(burn_2018))

#Subset burned areas to TNRD
burn_2018 = st_intersection(burn_2018, TNRD_conal)

#Now put the burn areas into UTM to match with the exported data
burn_2018 = st_transform(burn_2018, crs = crs(sf_2018))

#Find area of sample tiles
sf_2018$area = as.numeric(st_area(sf_2018))

#Subset grid data to only include grids that intersect polygons and the intersection area
##Note: problem still occurs when I use subset_18 = st_intersection(sf_2018, st_difference(burn_2018)) instead
subset_18 = st_intersection(sf_2018, burn_2018)

#Subset data so we only have the ID ("system.index")
subset_18 = subset_18[c(1)]

#check which rows of subset_18 are duplicated 
duplicated_rows <- subset_18[duplicated(subset_18$system.index), ]

#this number should be zero (there are four duplicated rows, they have different geometries but the same system.index)
duplicated_rows <- subset_18 %>%
  filter(system.index %in% c(duplicated_rows$system.index))

Here is the table of duplicated rows:

> duplicated_rows
Simple feature collection with 8 features and 1 field
Geometry type: GEOMETRY
Dimension:     XY
Bounding box:  xmin: -121.589 ymin: 50.24116 xmax: -119.7244 ymax: 51.99503
Geodetic CRS:  WGS 84
       system.index                       geometry
1670         2_3921 MULTIPOLYGON (((-120.2922 5...
2416         2_4770 POLYGON ((-119.7521 50.7734...
456          2_1766 POLYGON ((-121.5678 50.2530...
1738         2_3994 POLYGON ((-120.2048 51.9569...
1670.1       2_3921 POLYGON ((-120.2472 51.9927...
1738.1       2_3994 POLYGON ((-120.2454 51.9930...
2416.1       2_4770 POLYGON ((-119.7256 50.7660...
456.1        2_1766 POLYGON ((-121.5838 50.2439...