I have a construct in which I float a span (with original page numbers) in the right margin:

Fray Sibyla liet hem een figuur slaan als de visscher,
<a id="d0e290"></a><span class="pagenum">[<a href="#d0e290">8</a>]</span>die alen met strikken wilde vangen.

using the following stylesheet:

.pagenum
{
    display:inline;
    font-size:70%;
    font-style:normal;
    margin:0;
    padding:0;
    position:absolute;
    right:1%;
    text-align:right;
}

In a few cases, when the span in question appears near the line-break, the float gets rendered twice...

I can send a larger example of my HTML if desired.

Could you email me (mikeday@yeslogic.com) an example document that exhibits this behaviour?

This bug has been fixed in Prince 6.0 rev 3, available for download today. Thanks for reporting the issue!