Check for multiple occurence of OPF guide references with type="text"

[mkraetke]

For compatibility reasons, some of our clients request a guide in the OPF document. Within a guide, a reference with @type="text" indicates the first "real" content page, for example, the beginning of the chapter after the title pages. Our client GGP media noticed that you can add multiple references with @type="text" within a guide, resulting in an ambiguous indication of the so-called first "real" page. This might be a rare edge case, and most reading systems will simply use the first reference. So, I'll leave it up to you to decide whether you think EPUBCheck should check this.