Set key backup guard flag to avoid duplicate uploads
Set key backup guard flag to avoid duplicate uploads

Not sure how this was missed, but anyway, guard against duplicate uploads with
the existing flag.

matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,krombel/matrix-js-sdk,matrix-org/matrix-js-sdk,krombel/matrix-js-sdk,krombel/matrix-js-sdk,matrix-org/matrix-js-sdk