  It has been recently suggested that large solar energetic particle (SEP)
events are often caused by twin CMEs. In the twin-CME scenario, the preceding
CME is to provide both an enhanced turbulence level and enhanced seed
population at the main CME-driven shock. In this work, we study the effect of
the preceding CMEs on the seed population. We examine event-integrated
abundance of iron to oxygen ratio (Fe/O) at energies above 25 MeV/nuc for large
SEP events in solar cycle 23. We find that the Fe/O ratio (normalized to the
reference coronal value of $0.134$) $\leq2.0$ for almost all single-CME events
and these events tend to have smaller peak intensities. In comparison, the Fe/O
ratio of twin-CME events scatters in a larger range, reaching as high as $8$,
suggesting the presence of flare material from perhaps preceding flares. For
extremely large SEP events with peak intensity above $1000$ pfu, the Fe/O drop
below $2$, indicating that in these extreme events the seed particles are
dominated by coronal material than flare material. The Fe/O ratios of Ground
level enhancement (GLE) events, all being twin-CME events, scatter in a broad
range. For a given Fe/O ratio, GLE events tend to have larger peak intensities
than non-GLE events. Using velocity dispersion analysis (VDA), we find that GLE
events have lower solar particle release (SPR) heights than non-GLE events,
\red{agreeing with earlier results by Reames 2009b.
