Here the answer from Billy Lunt at the ARRL:
All digital modes count as "CW" for Field Day. If you work a station on
RTTY, then a CW contact on the same band would be a duplicate conatct.
> From: Will, KN6DV <kn6dv@QNET.COM>
> To: firstname.lastname@example.org; CQ-CONTEST <email@example.com>
> Subject: Field day RTTY--CW
> Here a part of the Field day rules.
> Question: does this means you can work a station on CW and RTTY or if you
> worked him on CW you can not work him again on RTTY? any band?
> I would like to set up a RTTY station for our local club.
> C.Each phone and each CW segment is considered as a separate band. All
> voice communication contacts are equivalent, and Packet/RTTY/ASCII/AMTOR
> counted as CW. A station may be worked once on each band. Crossband
> contacts are not allowed. The use of more than one transmitter at the
> time on a single band is prohibited, except that a Novice/Technician
> position may operate on any Novice band segment at any time. No repeater
> Unless the reformer can invent something which substitutes attractive
> virtues for attractive vices, he will fail.
> (Walter Lippmann)
> Submissions: firstname.lastname@example.org
> Administrative requests: email@example.com
> WWW: http://www.wf1b.com/
> Questions: firstname.lastname@example.org
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