Add import --exact. When this option is set, import will apply the patch (which must be generated by export) to the parents specified in the patch, and check that the node produced by the patch matches the node ID in the patch.

#!/bin/sh

mkdir a
cd a
hg init
echo 123 > a
hg add a
hg commit -m "a" -u a -d "1000000 0"

cd ..
mkdir b
cd b
hg init
echo 321 > b
hg add b
hg commit -m "b" -u b -d "1000000 0"

hg pull ../a
hg pull -f ../a
hg heads