Deferred index building not idempotent

I've got some demo scripts that I'm using to create a bunch of deferred indexes and then build them. Running this on a clean system works well, but when I was diagnosing issues and running the script repeatedly, I found that if any indexes already exist in the build index statement, the others are not built.

i.e.:
CREATE INDEX def_sourceairport ON `travel-sample`(sourceairport) USING GSI WITH

{"defer_build":true}

CREATE INDEX def_name_type ON `travel-sample`(name) WHERE (_type = "User") USING GSI WITH

{"defer_build":true}

BUILD INDEX ON `travel-sample`(`def_sourceairport`, `def_name`) USING GSI"

then
DROP INDEX `travel-sample`.def_name

then the same:
CREATE INDEX def_sourceairport ON `travel-sample`(sourceairport) USING GSI WITH

{"defer_build":true}

CREATE INDEX def_name_type ON `travel-sample`(name) WHERE (_type = "User") USING GSI WITH

{"defer_build":true}

(fails understandably)
BUILD INDEX ON `travel-sample`(`def_sourceairport`, `def_name`) USING GSI"
-this fails saying that def_name is already built, but def_sourceairport does not get built either-