Otherwise you can end up with different stores depending on how changes are merged in.

Case 1:
base.insert

{a, a}
other = base
me = base
other.update{a, b}
merge other to master = {a, b}
me.update{a, a}

merge me to master =

{a, b}

Case 2:
base.insert{a, a}
other = base
other.update{a, b}

merge other to master =

{a, b}

me = other
me.update

{a, a}
merge me to master = {a, a}

If the order is Case 1, update need to fail so we know there is nothing to merge in