The National Association of Realtors (NAR) will revise data on sales of existing homes going back to January 2007 because of improper counting, signaling that the housing market was weaker than previously reported.

Some homes were listed more than once in the NAR's data, leading to sales being recorded twice. Some new homes were also counted as previously occupied homes, the NAR said.

However, home prices will remain unchanged for the time period, and the monthly percentage change in volume should be relatively unaffected.

In February, real estate research firm CoreLogic said the NAR could have inflated home sales by up to 20 percent. The Nar subsequently revisited the data and held a benchmarking exercise.

The NAR usually revises previously reported sales based on U.S. Census data, but the 2010 Census didn't include information on home sales, so the NAR had to create a new model.

The NAR's new data will be released on Dec. 21.