  When calculating IMFs for young clusters, one has to take into account that
(a) most massive stars are born in multiple systems (b) most IMFs are derived
from data that cannot resolve such systems, and (c) multiple chance
superpositions between members are expected to happen if the cluster is too
distant. In this article I use numerical experiments to model the consequences
of those phenomena on the observed color-magnitude diagrams and the IMFs
derived from them. Real multiple systems affect the observed or apparent
massive-star MF slope little but can create a significant population of
apparently ultramassive stars. Chance superpositions produce only small biases
when the number of superimposed stars is low but, once a certain number
threshold is reached, they can affect both the observed slope and the apparent
stellar upper mass limit. I apply those experiments to two well known massive
young clusters in the Local Group, NGC 3603 and R136. In both cases I show that
the observed population of stars with masses above 120 solar masses can be
explained by the effects of unresolved objects, mostly real multiple systems
for NGC 3603 and a combination of real and chance-alignment multiple systems
for R136. Therefore, the case for the reality of a stellar upper mass limit at
solar or near-solar metallicities is strengthened, with a possible value even
lower than 150 solar masses. An IMF slope somewhat flatter than Salpeter or
Kroupa with gamma between -1.6 and -2.0 is derived for the central region of
NGC 3603, with a significant contribution to the uncertainty arising from the
imprecise knowledge of the distance to the cluster. The IMF at the very center
of R136 cannot be measured with the currently available data but the situation
could change with new HST observations. (abridged)
