Hi ynot,
No, there is no contradiction. A sotapanna’s understanding of aniccha, dukka, anaththa is at a lower level than a sakadagami’s, and hence a sakadagami should attain arahanth before a sotapanna.
In the puggala pannatti an eka biji is explained as a person who attains nibbana after one human bava. A sakadagami returns to this world (I think this refers to kama loka) once and attains nibbana there. A human “bava” is a longer time frame than a human “life time”. Hence a sakadagami would attain nibbana before a sotapanna.
The pali text for this section is as follows;
Katamo ca puggalo ekabījī? Idhekacco puggalo tiṇṇaṃ saṃyojanānaṃ parikkhayā sotāpanno hoti avinipātadhammo niyato sambodhiparāyano. So ekaṃyeva mānusakaṃ bhavaṃ nibbattetvā dukkhassantaṃ karoti—ayaṃ vuccati puggalo “ekabījī”.
Katamo ca puggalo sakadāgāmī? Idhekacco puggalo tiṇṇaṃ saṃyojanānaṃ parikkhayā rāgadosamohānaṃ tanuttā sakadāgāmī hoti, sakideva imaṃ lokaṃ āgantvā dukkhassantaṃ karoti—ayaṃ vuccati puggalo “sakadāgāmī”.
My understanding is that these are only broad classifications and not definite (that’s why kolankola says 2 OR 3 bhava). And the time frames refer to the maximum time period one is likely to take to attain arahanth, at a given point in time.
A sotapanna could attain a higher magga pala in this life itself. So at one point in time a person could be a sotapanna (either sattakkathuparam, kolankola or eka biji) but he could attain a higher magga pala in that life itself, which then makes the earlier classification (of him being a sattakkathuparam or kolankola or eka biji) invalid.