Fingerprint as duress would be an incredibly bad idea simply down to the fact that fingerprints don't operate as an exact science. They are implemented using a mathematical algorithm that alters the fingerprint into a digital template.
From AOSP documentation:
Biometrics offer a more convenient, but potentially less secure way of confirming your identity with a device. Under the tiered authentication model, primary authentication (that as, knowledge-factor based modalities such as PIN, pattern, and password) provides the highest level of security. Biometrics are in the secondary tier of authentication.
Even if the Fingerprint sensor and it's implementation meet the highest criteria for security (Class 3 (Strong)). These have an acceptance rate for False positives, Imposter Mimic, and Spoofing.
Class 3 STRONG rates are:
SAR of all PAI species: 0-7%
SAR of Level A PAI species:<=7%
SAR of Level B PAI species: <=20%
SAR of any individual PAI species <= 40% (strongly recommended <= 7%)
FAR: 1/50k
FRR: 10%
Therefore the margin of error not being nil the fingerprint reader and fingerprint duress would not be completely foolproof and secure.