The most damaging single weapon in the game is probably the relic sword Hades, provided that the opposition you are attacking is not immune to poison. If it is immune to poison, you do most damage with the relic spear Poseidon, and the best combination of weapons is Poseidon in your right hand and Hades in your left.

Now I'm tempted to re-start with a Sword+Spear setup for all characters. Is that still optimal for MM7 and MM8?

Also, is it possible to get more than one copy of a unique item (such as Hades) without hacking/trainers? My personal rule set is exploits-OK, hacking/trainers-not OK.

You can find at most 13 random Artifacts and Relics before the game stops providing them. Any one item can be found once and only once, so you can't get 4 Poseidons and 4 Hadeses without using an editor. The catch is that if you find and take any of the 5 placed Artifacts before you've taken 13 random ones, it counts against your allocation, so in practice, you can end up with any number of Artifacts and Relics from 13 up to 18.

The 5 paced Artifacts (the sword Excalibur, the dagger Mordred, the amulet Morgan, and the two rings Guinevere and Igraine) are found in specific places, and are provided independently of the random Artifacts and Relics, so these are the only items that you can legitimately have two of.

For ordinary weapons, I'm fairly sure that the sword/spear combination is optimum for melée combat in turn-based mode, but the Artifacts and Relics are different, so those need to be taken on their own merits. However, in this game, and I'm pretty sure in MM7 as well, the best weapon in real-time melée combat is unquestionably the blaster. It doesn't do much damage in any one shot, but it fires so quickly that the damage done over a given time-period outweighs any other weapon. But they're useless in turn-based mode.

Having said that, I still consider that the most effective combat technique in MM6 is the spell Shrapmetal in contact range, cast by a fully developed Dark Magic master equipped with Guinevere and a Ring of Dark Magic. Each of those multiplies the effectiveness of the spell by 1.5 (with the obtained damage figure rounded down if it's not a whole number).

However, in the v1.0 game, you can get the benefits for leather and chain by taking the appropriate skill level in chain and plate respectively. Once my characters have reached master level in these two skills, I consider it legitimate to use an editor to confer mastery in chain on the leather master, and mastery in plate on the chain master. That way, I get the recovery benefits. I don't, of course, equip the leather user with chain or the chain user with plate.

Therefore, spear skill only increases damage and armor class, not attack. Axe skill only increases damage, not attack and recovery time. Sword skill (for the sword in off-hand) increases attack and decreases recovery time.

So, when dual-wielding, invest in both skills equally to get the best DPS.

Sidenote 1: it's different in MM7 and 8, where spear skill only effects armor class while sword skill effects damage, recovery time, and attack so clearly it's wrong to invest in spear skill in those games.