\starttext

\placeregister[index]

x \index{b} x
    \startregister   [index][x]{bb}
        x \page x \page x \page x
    \stopregister       [index][x]

x \index{c} x
    \setregisterentry[index][label=x,entries=bb]
        x \page x \page x \page x
    \finishregisterentry[index][label=x]

x \index{d} x
    \setregisterentry[index][label=y]
        x \page x \page x \page x
    \finishregisterentry[index][label=y,entries=yy]

\stoptext
