diff --git a/docs/architecture/scripts/preflight.sh b/docs/architecture/scripts/preflight.sh
index c99787c..c1f6343 100755
--- a/docs/architecture/scripts/preflight.sh
+++ b/docs/architecture/scripts/preflight.sh
@@ -49,6 +49,14 @@
 # The active checks are further gated behind --auth, so the default run sends
 # exactly two GETs and could not be mistaken for anything.
 #
+# ## Present is not in force
+#
+# It also counts each header rather than only looking for one. More than one
+# copy means the later ones are discarded — RFC 6797 requires exactly that for
+# HSTS and browsers do the same elsewhere — so a directive can sit in a response,
+# be read by a person as applying, and have never once applied. Two layers each
+# adding their own is all it takes.
+#
 # ## What it cannot tell you
 #
 # A header being present is not a header being correct — a CSP of
@@ -150,13 +158,49 @@ HEADERS=$(curl -sS -I -L --max-time 20 "$TARGET" 2>/dev/null) \
 
 lower_headers=$(printf '%s' "$HEADERS" | tr '[:upper:]' '[:lower:]')
 
+# `curl -L` concatenates the headers of every response in the chain, so a header
+# counted across all of them would report a redirect's copy as a duplicate of
+# the final page's. Only the last response block is counted.
+final_block() { awk 'tolower($0) ~ /^http\// { buf = "" } { buf = buf $0 "\n" } END { printf "%s", buf }'; }
+final_headers=$(printf '%s' "$lower_headers" | final_block)
+final_raw=$(printf '%s' "$HEADERS" | tr -d '\r' | final_block)
+
+# Present is not the same as in force. RFC 6797 section 8.1 is explicit for HSTS
+# -- more than one and the agent MUST process only the first -- and browsers
+# behave the same way for the others: the second copy is discarded in silence.
+# Two layers each adding their own is the ordinary cause, and the result reads
+# to a person as though both applied.
+#
+# Found on this script's first real target: an origin serving
+#   strict-transport-security: max-age=63072000; includeSubDomains
+#   strict-transport-security: max-age=63072000; preload
+# where preload had never once been in force.
+check_duplicate() { # <header-name>
+  local name="$1" count first
+  count=$(printf '%s' "$final_headers" | grep -cE "^${name}:" || true)
+  [ "${count:-0}" -gt 1 ] || return 0
+  # Counted against the lowercased copy, but quoted from the original: echoing
+  # `includesubdomains` back at somebody who wrote `includeSubDomains` reports a
+  # value they did not send. Directive names are case-insensitive; the report
+  # should still show what is actually on the wire.
+  # The FIRST occurrence, within the FINAL response block. Both halves matter:
+  # RFC 6797 processes the first and discards the rest, and `curl -L` hands us
+  # the headers of every hop, so searching the whole buffer would quote a
+  # redirect's copy. Quoted from the original rather than the lowercased copy —
+  # echoing `includesubdomains` at somebody who wrote `includeSubDomains`
+  # reports a value they never sent.
+  first=$(printf '%s' "$final_raw" | grep -iE "^${name}:" | head -n 1 | sed -E "s/^[^:]*:[[:space:]]*//")
+  finding "${count} ${name} headers — only the first is processed, so what is in force is '${first}' and every later copy is discarded silently"
+  return 1
+}
+
 case "$TARGET" in
   https://*) ok "the target is https" ;;
   *) finding "the target is not https — everything in transit is readable, including the session cookie" ;;
 esac
 
 if printf '%s' "$lower_headers" | grep -q '^content-security-policy:'; then
-  ok "content-security-policy present"
+  check_duplicate content-security-policy && ok "content-security-policy present"
 else
   finding "no content-security-policy header — injected script has nothing to stop it"
 fi
@@ -165,13 +209,13 @@ fi
 # and x-frame-options the one older browsers read, so one of the two is enough.
 if printf '%s' "$lower_headers" | grep -q '^x-frame-options:' \
   || printf '%s' "$lower_headers" | grep -q 'frame-ancestors'; then
-  ok "framing policy present"
+  check_duplicate x-frame-options && ok "framing policy present"
 else
   finding "neither x-frame-options nor a csp frame-ancestors — the page can be framed and clickjacked"
 fi
 
 if printf '%s' "$lower_headers" | grep -q '^strict-transport-security:'; then
-  ok "strict-transport-security present"
+  check_duplicate strict-transport-security && ok "strict-transport-security present"
 else
   finding "no strict-transport-security — the first request of each visit can still be plaintext"
 fi