Eclipse birds can be very confusing (thanks Alan for doing what I should have done, explaining what eclipse plumage is), the birds below could easily be mistaken as "a pair" but are both drakes in different stages of moult.
Another pitfall at this time of year are some moulting female Mallards, they often have pale feathers which look white at any distance. This could cause cofusing them with Gadwall so always check the bill pattern and colour (see image below)
Below is a Gadwall, note bill pattern and colour
Another example of an eclipse drake duck is this Shoveler below
hope these are of interest
cheers, John
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