Voting at the Hiawatha David Jr. Rec. Center polling station in Denver's Park Hill neighborhood on Election Day 2016.

(Hart Van Denburg/CPR News)

It appears not all of Colorado's unaffiliated voters have gotten the message about the rules to participate in the primary election.

Under the state's new open primary system, unaffiliated voters received both  Democratic and Republican ballots in the mail. They can only return one for their vote to count.

As of last week, around 7 percent of unaffiliated voters who returned ballots in El Paso County incorrectly sent in both. Other large counties are reporting lower rates. In Arapahoe County, 4.3 percent of unaffiliated voters submitted both ballots, in Denver, 3.4, and in Larimer county, 3.15.

As of last Thursday, 234,065 Colorado voters had returned their ballots. About 3,600 more Republicans had voted than Democrats. Of the 49,661 unaffiliated voters who'd cast ballots, 12,655 voted in the Democratic primary while 9,261 chose Republican.

Not all clerks are opening unaffiliated ballots immediately however -- 27,745 were still unopened at county offices as of last week.

Counties are using different techniques -- from instruction booklets to warnings on the secrecy sleeve -- to try to keep unaffiliated voters from returning more than one ballot. The Secretary of State's office is also running a public information campaign, UChoose, to try to educate voters on the process.