Parallel mode fails with multiple -L inputs

[ZeLonewolf]

When merging multiple GeoJSON in parallel mode using -L, I get weird errors like Found ] at top level: and the resulting PMTiles drops most of the input data (only one layer actually had data.

Sample command:

tippecanoe --force -P --no-tile-size-limit -o out.pmtiles --single-precision --extend-zooms-if-still-dropping --buffer=0 \
-L'{"file":"merged_1.geojson", "layer":"layer1", "description":"Layer 1"}' \
-L'{"file":"merged_2.geojson", "layer":"layer2", "description":"Layer 2"}' \
-L'{"file":"merged_2.geojson", "layer":"layer3", "description":"Layer 3"}'