diff --git a/build.ps1 b/build.ps1
index 66490c1..ab0a194 100644
--- a/build.ps1
+++ b/build.ps1
@@ -156,9 +156,19 @@ function Invoke-Swift([string[]]$SwiftArgs, [hashtable]$Sql) {
     # depends on clang honouring CPATH — if it didn't, the failure would be an inscrutable
     # "'sqlite3.h' file not found" from inside GRDB's module build. -Xlinker only matters
     # when something actually links (tests, executables); a library build needs just the -I.
+    #
+    # TEMPORARY — /FORCE:MULTIPLE (docs/WINDOWS_PORT.md §14.1.1). swift-crypto's
+    # CCryptoBoringSSL and swift-nio-ssl's CNIOBoringSSL both land in every Windows link, and
+    # `p_thread_callback_boringssl` (crypto/thread_win.cc) collides between them: BoringSSL's
+    # prefix maps are generated by building on a POSIX host, so symbols from the Windows-only
+    # translation units are never prefixed. The same pair links clean on Linux, where the
+    # equivalent file is thread_pthread.c and IS prefixed. Drop this flag the moment one copy
+    # carries the prefix — it is global, so it also downgrades any FUTURE duplicate to a
+    # warning. Read the warnings.
     $full = @($SwiftArgs) + @(
         '-Xcc', "-I$($Sql.Include)",
-        '-Xlinker', "-LIBPATH:$($Sql.Lib)")
+        '-Xlinker', "-LIBPATH:$($Sql.Lib)",
+        '-Xlinker', '/FORCE:MULTIPLE')
     Write-Note "swift $($full -join ' ')"
     # Out-Host, not the pipeline: a native command's stdout would otherwise become this
     # function's return value, and the caller's exit-code check would see an array.