I recently installed QGIS 2.18.3. I loaded some WFS layers (1.1.0) I have previously loaded using 2.14.9 and noticed the record count had doubled (and one time they trippled but I can't replicate that). The duplication was consitent with a few layers I checked from the same URL.
I tried the same WFS URL using 2.18.0. I have one layer with just one point feature and 2.18.0 returns only one feature however, in other layers 2.18.0 seems to randomly duplicate features and drops other features. For example, 2.18.0 returns 1217 features in a layer with 1220 features - some missing and some duplicated.
I tried a 'select distinct * from table' query using 2.18.3 and get 1186 features returned (2372 without distinct). I can't see any duplicate values but there are obviously some missing.
Are there issues with WFS in 2.18.3?
Further information: I have confirmed that the WFS is being served with Geoserver 2.7.0. I have one layer that has an ID (it is not titled FID) that is unique. The IDs range from 1 to 48 (I'm assuming this meets the requirement of a WFS FID??). If I use ArcGIS 10.4 Interoperability extension, QGIS 2.6.0 or QGIS 2.18.0 then I get the correct number of features. If I use 2.18.3 then I get exactly twice the number of records. It seems the request is made twice and I get one record set appended to another.