Formatting changes when simply duplicating a text box

I'm not sure if anybody has had an issue with this - I've never experienced it before - but when I simply duplicate a text box, the formatting changes in the dupicated box. I've attached a screen grab - the text box on the right is a straight duplicate and as you can see the text is sitting lower in the box and overhanging the margin line.

The same thing also happens to this one and one other text box on the page even if I duplicate the spread or copy them from on page to another. 

It seems like a bug as I can't even locate what kind of formatting would do this?

The odd thing is that this is only happening on my iMac at home - there are no issues with the very same InDesign file on my iMac at work or on my MacBook Pro - same system software, same version of InDesign.

I've even removed an re-installed InDesign and deleted my preferences and still the same issue.
It's really frustrating and rather worrying, as it's part of the artworking of a large architectural monograph and it's put some rather worrying doubts in mind!