'DefaultValue' property was already registered by 'Xceed_Wpf_Toolkit_NumericUpDown`1_12_151463089'

Question

Error Description

I've searched for this error but every other topic was involving C# code and metadata/dependency overridings.

But my problem actually involves only and just XAML declarations (of course, it has "MyControl.xaml" and "MyControl.xaml.cs", but nothing that even mention any control that gives those errors).

Also, it does compile, the program runs OK, everything is fine, but it gives a build error (tracked in blue) is annoying to see every line envolving xctk underlined in blue as if it had something wrong.

If I do:

<xctk:IntegerUpDown />

I get this:

'DefaultValue' property was already registered by 'Xceed_Wpf_Toolkit_NumericUpDown'1_12_151463089'

How to reproduce

Just do anything envolving Xceed WPF Toolkit:

<xctk:DecimalUpDown />
<xctk:IntegerUpDown x:Name="something" />
<xctk:IntegerUpDown Name="something" />
<xctk:IntegerUpDown DefaultValue="0" />
<xctk:IntegerUpDown x:Name="something" DefaultValue="0" />

No aditional C# code involved, still all of above gives the same error.

Aditional Information

"xctk" is defined this way in my UserControl XAML:

<UserControl
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    ...other definitions...
    xmlns:xctk="http://schemas.xceed.com/wpf/xaml/toolkit"

What could it be related with and what should I check first?

(EDIT)

More Aditional Information

Just found that DoubleUpDown doesn't raise the error:

<xctk:DoubleUpDown />