diff --git a/compiler/cgen.nim b/compiler/cgen.nim
index a1b36c2db1..37b55899b9 100644
--- a/compiler/cgen.nim
+++ b/compiler/cgen.nim
@@ -1379,6 +1379,13 @@ proc genProcBody(p: BProc; procBody: PNode) =
 proc genProcLvl3*(m: BModule, prc: PSym) =
   if m.config.cmd == cmdNifC:
     fillBackendName(m, prc)
+    if (prc.disamb and (InstanceDisambBit or HookDisambBit)) != 0'i32 and
+        containsOrIncl(m.emittedContentDefs, stripCnifMarks(prc.loc.snippet)):
+      # A different symbol already emitted a body under this content-addressed
+      # C name in this TU (same generic instance / hook minted in two source
+      # modules, both loaded here). Emitting a second body is a C redefinition;
+      # a prototype was already produced for it, so just stop.
+      return
     if sfDispatcher in prc.flags and sfMainModule notin m.module.flags:
       # A method dispatcher enumerates the whole program's method set: its
       # body is synthesized by `generateIfMethodDispatchers` only after all
@@ -2533,6 +2540,7 @@ proc rawNewModule(g: BModuleList; module: PSym, filename: AbsoluteFile): BModule
   result.headerFiles = @[]
   result.declaredThings = initIntSet()
   result.declaredProtos = initIntSet()
+  result.emittedContentDefs = initHashSet[string]()
   result.icImplMods = initIntSet()
   result.cfilename = filename
   result.filename = filename
diff --git a/compiler/cgendata.nim b/compiler/cgendata.nim
index ca34f8e870..8e096d4f00 100644
--- a/compiler/cgendata.nim
+++ b/compiler/cgendata.nim
@@ -158,6 +158,12 @@ type
     forwTypeCache*: TypeCache # cache for forward declarations of types
     declaredThings*: IntSet   # things we have declared in this .c file
     declaredProtos*: IntSet   # prototypes we have declared in this .c file
+    emittedContentDefs*: HashSet[string]
+      # cmdNifC per-module backend: content-addressed C names (generic
+      # instances and synthesized hooks) whose body this TU already emitted.
+      # Distinct symbols (minted in different source modules) can share one
+      # `_i<disamb>` name; `declaredThings` keys on symbol id and lets the
+      # second one through, so we dedup the body by name here instead.
     queue*: seq[PSym]         # queue of procs to generate
     alive*: IntSet            # symbol IDs of alive data as computed by `dce.nim`
     headerFiles*: seq[string] # needed headers to include
diff --git a/compiler/deps.nim b/compiler/deps.nim
index 07e4652673..3ec648283f 100644
--- a/compiler/deps.nim
+++ b/compiler/deps.nim
@@ -34,6 +34,9 @@ type
     processedModules: Table[string, int]  # modname -> node index
     includeStack: seq[string]
     systemNodeId: int  # ID of the system.nim node
+    scanningMain: bool # currently scanning the project main module's deps;
+                       # makes `when isMainModule` conditions evaluate true
+                       # only there (every other module is imported)
 
 proc toPair(c: DepContext; f: string): FilePair =
   FilePair(nimFile: f, modname: moduleSuffix(f, cast[seq[string]](c.config.searchPaths)))
@@ -256,6 +259,14 @@ proc evalCondIdent(c: DepContext; v: string): bool =
     # defined(gcHooks)`; guards mmdisp.nim's `include "system/gc"` whose
     # transitive imports (sharedlist, locks) an orc compile never produces.
     isDefined(c.config, "gcDestructors") or isDefined(c.config, "gcHooks")
+  of "isMainModule":
+    # Only the project main module is compiled with `isMainModule` true; an
+    # imported module's `when isMainModule` blocks are dead. The conservative
+    # `true` would schedule main-only imports (e.g. parser.nim's
+    # `tools/grammar_nanny`, a node that gets a cg rule but is never linked,
+    # so the merge stage can pick it as a shared def's owner -> undefined
+    # symbols at link).
+    c.scanningMain
   else: true
 
 proc evalCondExpr(c: DepContext; s: var Stream): bool =
@@ -494,6 +505,12 @@ proc readDepsFile(c: var DepContext; pair: FilePair; current: Node) =
   if not fileExists(depsPath):
     return
 
+  # `current.id == 0` is the project main (rootNode); restored on exit so the
+  # flag is correct for each parent frame between its child recursions.
+  let prevScanningMain = c.scanningMain
+  c.scanningMain = current.id == 0
+  defer: c.scanningMain = prevScanningMain
+
   var s = nifstreams.open(depsPath)
   defer: nifstreams.close(s)
   discard processDirectives(s.r)
diff --git a/compiler/nifbackend.nim b/compiler/nifbackend.nim
index 62a873393f..dae0ea67f1 100644
--- a/compiler/nifbackend.nim
+++ b/compiler/nifbackend.nim
@@ -154,6 +154,14 @@ proc signatureHasMetaType(t: PType; depth: int = 0): bool =
   ## element case, hence the explicit scan.
   result = false
   if t == nil or depth > 8: return false
+  if t.kind == tyGenericBody:
+    # The uninstantiated template carried as a `tyGenericInst`'s first child
+    # always mentions its `tyGenericParam` placeholders, but the instance
+    # itself is fully concrete (e.g. `var CountTable[SigHash]`). Descending
+    # here would wrongly flag every routine with a generic-instance parameter
+    # as meta and drop it from the owned-routine seeding -> undefined symbols
+    # at link (its only definer never emits it).
+    return false
   if t.kind in {tyTyped, tyUntyped, tyTypeDesc, tyStatic, tyGenericParam,
                 tyAnything, tyFromExpr, tyError}:
     return true