Because D.C. is a one-party town, the Congress created two at-large City Council seats with the intent of creating at least one minority party member. Every voter gets two votes. The catch is that each party can only nominate one candidate. For years, this worked, as a Republican won the second seat. However, in the last election, a Democrat running as an Independent won. Thus, the City Council will be completely Democratic come January. This completely defeats the intent of this voting rule.
The big question is what took the Democrats so long? They could have always designated an independent as the second Democratic candidate.
The next big question is how to fix it? Two methods come to mind. The first is the Japanese method for multimember districts: give voters only one vote. The problem is that doesn't work for the same reason. The Democrats can nominate two candidates, divide D.C. into two halves and instruct voters in each half to vote for that half's candidate. Thus, the Democrats can hold the two cities. The other alternative is for each party to designate an at-large candidate. Then, the two parties receiving the highest number of votes for all other council seats receive one vote each. In a city dominated by one party, this will have the desired effect of placing a minority party member on the City Council. If the city becomes politically competitive, the effect is a wash, as the balance between the two parties will not be affected.