In Final Fantasy Tactics for the original PlayStation, you can use the "Move-Find Item" ability to find hidden items. If you find the common item on a particular tile, can you play through that map again and have a chance of finding the rare item on that tile? Or can you only find an item once on a tile?
You can farm items that have a rare chance as long as you have unlimited map access (i.e. Dorter Trade City fight only happens once, so you can only get items from there once).
Here's a guide complete with the drop rates. Maps labelled with OTB mean that it is a one-time battle.
After playing through a few Deep Dungeon maps multiple times, it appears you can only acquire an item once from a tile with "Move-Find Item". Attempting to find an item a second time in a subsequent playthrough will trigger a trap. This is supported by A Knightblade's Guide to Finding Items,
In some cases, an item may be 'buried' beneath another item. This is often the case in the Deep Dungeon, or with the best items. If you get the 'poor' quality item, you've lost the chance to get the superior one.