When the sequence <cc> is followed by <e> or <i> the pronunciation is ks. Examples:
accept accident occidental succeed vaccine
Elsewhere the pronunciation is k, as in:
acclaim accord account accrue accuse occur raccoon succour
The word flaccid has two competing pronunciations: ˈflæksɪd and ˈflæsɪd.
Thanks to Petr Rösel for adding the medical term coccyx = ˈkɒksɪks