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Thanks for the E-mail Dusan! The deck looks good, but a few things do jump out at me.
Number one, you said you have problems with Dredge creatures, but you already have an answer in the deck: Dimir Doppelganger itself. The Doppelganger’s ability removes the target card in ANY graveyard from the game, so you could use it on your opponent’s upkeep to copy whatever card you don’t want him/her to Dredge back. Of course, if the Doppelganger is already something better, than I could see where you’d want another answer. Number two, I noticed you have exactly three copies of every nonland card in the deck. This is fine, but awfully strange. You do realize that you’re allowed up to four of each card that isn’t a basic land, don’t you? Well, maybe we’ll go up to four on some cards. And number three, it sounds from your letter like your main use for Blackmail is to target yourself? I’m sorry, but that’s just not good strategy. If you want to discard cards yourself, discard them as a cost to get something back, like with Thought Courier. With him, you discard a card and get to draw a new card. Blackmail is still a good card, but target an opponent with it please.

Okay, right now you’ve got nine counterspells, but quite frankly I’m not a big fan of Rewind. Yes, it’s free, but only if you leave four mana open. That’s a lot of mana to leave open, and if your opponent doesn’t play anything threatening, you’ll often regret not using that mana for something else. Let’s take out Rewind, and add an additional copy each of Mana Leak and Perplex. Mana Leak is a great counter. Perplex not so much really, but I’m adding another copy because I like it’s transmute ability. Perplex can search your deck for Dimir Doppelganger, Rend Flesh, or Royal Assassin. And maybe I’ll add in some more cards that cost 3 for Perplex to tutor up. Another card that I have reservations about is Hideous Laughter. The only creature you have that can survive it is Mindleech Mass, or a Dimir Doppelganger that copied a creature that can survive it. That can go. As far as keeping your opponent’s board clear goes, I suggest a fourth Last Gasp, and a Ribbons of Night or two. Everything else you’ve got should hopefully be enough.
Now, for an answer to Dredge creatures. The best bet is to be able to remove cards from your opponents’ graveyards. But against any non-Dredge deck, that card will be useless unless it does other things. My solution: Junktroller. Its 0/6 body will hold off opposing creatures that got past your counterspells, discard, and removal while its ability can take dead Dredgers and move them to the bottom of your opponent’s library where they’ll be of no threat.
And speaking of Dredge, since you seem to be in such a hurry to get Mindleech Mass in the graveyard, Dredge seems the perfect fit. But which Dredge card? A black one obviously, and hopefully one that costs three mana, so Perplex can go find it. That narrows it to Stinkweed Imp and Necroplasm. Necroplasm’s great, but for your deck I’d go with the Imp. It gives you a flying creature that can deal with Kokusho and keep coming back. But I’ll only add one, since your four Perplexes effectively act as copies of it if necessary.

So, what have I done so far?
-3 Rewind
-3 Hideous Laughter
+1 Mana Leak
+1 Perplex
+1 Last Gasp
+2 Ribbons of Night
+2 Junktroller
+1 Stinkweed Imp

I added two more cards than I put in. Well, what can we drop? Well, I think the best way is to drop Lore Broker altogether, and add one more copy of Dimir Guildmage. Yes, Dimir Guildmage needs a bit of mana to be good, but he doesn’t make you discard when you draw, and he doesn’t give your opponent cards. Thought Courier is nice, but Lore Broker and Thought Courier in the same deck is a bit much. So, the final resulting decklist looks like:

Creatures:
3 Dimir Doppelganger
3 Royal Assassin
4 Dimir Guildmage
3 Thought Courier
2 Junktroller
1 Stinkweed Imp
3 Mindleech Mass

Other spells:
4 Mana Leak
4 Perplex
3 Blackmail
2 Ribbons of Night
4 Last Gasp
3 Rend Flesh

Land:
11 Swamp
10 Island

Of course, it’s your deck, and if you disagree with me, that’s your right. Just to get you thinking, here are some other cards I wanted to put in and didn’t have room for:
Compulsive Research
Hunted Phantasm
Spawnbroker(combo with Last Gasp)
Hissing Miasma
Reminisce
Underground River
Dimir Signet
Dimir Infiltrator

That should be plenty to think about for now. It is for me anyway. Best of luck to you, Dusan!