2016 Congressional Margin of Victory
Districts where first and second place were from the same political party
The margin of victory calculated by Ballotpedia is based on the percentage difference between the two candidates who received the most votes. In most cases that is a Democrat and a Republican, although in a few cases a 3rd party or independent candidate came in second.
However, in 12 cases, the margin was between two candidates of the same party. This can occur in California and Washington, where non-partisan primaries are held. The top two vote-getters, regardless of party, go on to meet in the general election. It can also happen in Louisiana, where all candidates, regardless of party, run in the general election. If nobody receives over 50% of the vote, there is a runoff between the top two.
We point this out only because the margin of victory in some of these races is quite a bit different than it would have been in a 'traditional' general election. For example, California's 44th district is one of the most Democratic-leaning in the country. Hillary Clinton won here by nearly 71% over Donald Trump. However, the two Democrats that met in the House race on Election Day were only separated by 4%.
Here are the 12 Districts
Two Democrats: California 17,29,32,34,37,44,46; Louisiana 2; Washington 7
Two Republicans: Louisiana 3,5; Washington 4