I came across this article by Alan Sharp and thought it was interesting. It refers to Mailing your database with offers, actually the same offers twice. This article was written with the assumption that you are mailing in quick succession or at the same time.
Mailing the same direct mail offer to the same consumer list a second time typically generates a response rate that's 65 percent smaller than the initial response. Mailing a third time usually generates a response that's more than 50 percent smaller than the initial mailing. But if you mail businesses or institutions with the same offer more than once, your results sometimes run the other way.
Some business-to-business direct marketers have discovered that the same offer mailed to the same list a second time produces double the response of the initial mailing.
Hard to believe, I know. But this just proves that business buyers and consumers are different.
The business executive you tried to reach with your first mailing may have been lying on a beach in the Seychelles when your offer arrived. Or her secretary may have pitched it.
Or the guy in the mailroom may have had a bad day and routed your direct mail offer to Bangladesh.
Or your prospect may have suffered a financial setback that resolved itself by the time your second offer arrived in his inbox.
Or the timing may have been off. Your prospect was not ready to buy last quarter but is ready this quarter.
These reasons, and many more, should encourage you to test mailing your direct mail sales letters to the same prospects more than once. Keep everything the same (list, offer, creative). Just vary the timing. Measure your response between mailing one and mailing two, and even mailing one and mailing three.
Keller William's Agents know from the Millionaire Real Esate Agent that a consistent mailing is the most successful and effecient route to market to a "Farm" area. Spread out over a year, you should be able to realistically net 1 response per 50 mailings. Much better than the 1 per 1000 that we've heard. (For those at home doing the math, that would be 600 total mailings. Or rather 1 in 600).
This study shows that sending the same offer out to a Realtor's Farm in close proximity won't really increase the effectiveness, since it's rare for Buyers or Sellers to change their mind about a buying/selling decision in the matter of days. Typically, there is a 12 to 9 month buying cycle.

"Some times you feel like a nut, some times you don't." And I'm not talking about a candy bar. 8-)
Some days I read/glance at every e-mail and letter I receive. But when my mother had a recent stroke, I didn't look at anything! One business associate that knew I always respond even if I can't attend called. She knew me well enough to question why I didn't respond. I would have missed a great networking event.
So Joshua, you are right about multiple contacts. You never know what is going on in another person's life. And in some cases it may take many contacts to get peoples' attention.
Have a great day!
Thanks for all the great responses. Just a word of note though, this study and article are in regards to DIRECT MAIL, not e-mail.
Gary - That's pretty funny. I'm assuming that you are being a bit cynnical here. The study meant the same exact offer in close succession, not necessarily on the same day. I'd probably see it as the same though, a mistake. Mistakes don't always equal higher costs to the consumers though.
Michael - I agree. If I see the same offer, especially the same EXACT offer then I assume it was a mistake somewhere.
Russ - Everything counts as a touch, that's for sure. However, mailing so close in proximity might be counter-effective.
Scott - I've always got ears (or eyes) for your advice! I'm sure there are strategies out there for dual mailings or e-mailings. Even a "planned" mistake to get the reader to see what the difference is could be an effective strategy. Especially with e-mail.
Yeah, I have to agree with this. Sometimes there are things that I would like to do, but I am holding off because at the time it may not fit into my overall monthly marketing budget. As I free up capital I may be more inclined to pay for a service or product that could have a positive impact on my business.