Can the duration of Gentle Repose be extended by multiple castings?

The paladin in my group just died, and we want to bring him to an archdruid to reincarnate him. The group needs 12 days to reach the archdruid, so in this case one casting of Gentle Repose is enough to keep the body well preserved.

However, I was wondering if you can cast Gentle Repose multiple times in order to extend the “revivability” of a body as much as you like. Is this possible? Does this make a character unkillable?

By “unkillable” I mean that, even if a 1st level character dies, his friends just need to cast Gentle Repose once in a while until the cleric reaches level 9 (or the druid level 7) and can resurrect/reincarnate him. I’ve always been against Resurrection/Reincarnation spells because they make death as lethal as a cold in mid levels, but this spell effectively extends the “unkillability” of a character way down to level 1. Not having a temporal window in which a character can be resurrected, right from level 1, takes away some steam to the game and the only way to make an adventure exciting is only through encounters where TPK can happen.