pack-bitmap: handle duplicate pack entries during MIDX reuse #37

Job	Run time
config	6s
win+Meson build	0s
win+VS build	0s
fuzz smoke test	0s
sparse	0s
static-analysis	0s
${{ matrix.vector.jobname }} (${{ matrix.vector.pool }})	0s
${{ matrix.vector.jobname }} (${{ matrix.vector.image }})	0s
win build	0s
win+Meson test	-1s
rust-analysis	-1s
win test	-1s
documentation	-1s
win+VS test	-1s
	1s

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