openCover reporting unexpectedly low coverage

Question

I have the following code:

var isDuplicateCategoryIds = _query.Categories
    .GroupBy(x => x.Id)
    .Any(g => g.Count() > 1);

I have two tests that invoke this code, one where there are duplicate Ids and one where there are not.

However, openCover (which is used on our CI server) is reporting:

Partially covered by tests (16 of 44 conditions).

Locally, dotCover is reporting 100%.

As I understand it, openCover uses the branches & conditions generated in the IL, and the IL for this statement is the following 24 lines (named symbols removed):

IL_0001: ldarg.0      // this
IL_0002: ldfld        
IL_0007: callvirt     
IL_000c: ldsfld       
IL_0011: dup
IL_0012: brtrue.s     IL_002b
IL_0014: pop
IL_0015: ldsfld       
IL_001a: ldftn        
IL_0020: newobj       
IL_0025: dup
IL_0026: stsfld       
IL_002b: call         
IL_0030: ldsfld       
IL_0035: dup
IL_0036: brtrue.s     IL_004f
IL_0038: pop
IL_0039: ldsfld       
IL_003e: ldftn        
IL_0044: newobj       
IL_0049: dup
IL_004a: stsfld       
IL_004f: call         
IL_0054: stloc.0      

My question is how do I get coverage here?

As an aside: Is it even worthwhile to attempt do so? I have tests covering the two scenarios this code is concerned with; dupes & no dupes. There is code executed prior to this that checks whether Categories is null/empty & under that scenario this code won't be invoked. So if some of these conditions being generated under the hood are for that (I don't know that they are), it's a moot point.