There are certain abilities in magic the gathering that sound cool when you see them but as you begin to examine them you recognize that they are generally not as powerful as they seem. This is usually because of some restriction that makes them far too situational to be used often making them something that sits in your hand or on the board doing nothing because you simply have no reason to use it.
Twinning glass is a rare artifact f rom the lorwyn expansion set of magic the gathering. It costs four colorless mana to put into play and one colorless mana and tapping to activate its ability which reads "You may cast a nonland card from your hand without paying its mana cost if it has the same name as a spell that was cast this turn."
This seems at first to be a reasonable ability, after all you are playing four copies of every card in your deck so you are going to have two copies of the same spell often enough that this is likely to be of some use. But you need to think about it a bit more carefully.
The first problem is that this is four mana to put into play, that means without some type of mana acceleration you won't have it into play until the forth turn. This means that in most decks you will have played out at least half of your hand so now rather than having seven cards you have three or four. This lowers the odds of two of them being the same considerably.
Next is the problem of land, you can't use this to play land, which is a third of your deck, lowing the odds of it working any more. This takes it down to about forty card that might be useable with this.
From there we get into the things you are less likely to think of. The first point is your one drop spells. These are generally pointless to use with the twinning glass since the cost is the same. Most decks which want to be quick have at least two sets of these which is eight cards dropping the spells you'll want to use this with to a maximum of 32.
Next are spells you won't want to copy. These are spells like wrath of god and often but not always counter spells. Any enchantment that effects everyone permanently and other spells. Assuming that the average deck has two sets of cards that fit this category this make the potential cards twenty four. Assuming that every one of these is a card you would want to cast and you draw two of them you are still using an extra card to get it out a turn earlier and the odds of using this twice in a game go down even more. It is just not worth it.