diff --git a/python/fasttext_module/fasttext/tests/test_quantize_twice.py b/python/fasttext_module/fasttext/tests/test_quantize_twice.py
new file mode 100644
index 0000000..d466821
--- /dev/null
+++ b/python/fasttext_module/fasttext/tests/test_quantize_twice.py
@@ -0,0 +1,42 @@
+# SPDX-FileContributor: Arthit Suriyawongkul
+# SPDX-FileCopyrightText: 2026-present, fasttext-community
+# SPDX-FileType: SOURCE
+# SPDX-License-Identifier: MIT
+
+"""quantize() must raise on a quantized model, and only on one."""
+
+import pytest
+
+from .helpers import build_supervised_model, get_random_data
+
+
+def _model():
+    # thread=12: thread <= 10 leaves the input matrix partly uninitialized.
+    data = get_random_data(3000, max_vocab_size=600)
+    return build_supervised_model(data, {"thread": 12, "dim": 16, "verbose": 0})
+
+
+def test_quantize_twice_raises():
+    model = _model()
+    model.quantize()
+    with pytest.raises(ValueError, match="already quantized"):
+        model.quantize()
+
+
+def test_load_model_clears_quantized(tmp_path):
+    model = _model()
+    path = str(tmp_path / "model.bin")
+    model.save_model(path)
+    model.quantize()
+    model.f.loadModel(path)
+    assert not model.is_quantized()
+    model.quantize()
+
+
+def test_set_matrices_clears_quantized():
+    model = _model()
+    matrices = model.get_input_matrix(), model.get_output_matrix()
+    model.quantize()
+    model.set_matrices(*matrices)
+    assert not model.is_quantized()
+    model.quantize()
diff --git a/src/fasttext.cc b/src/fasttext.cc
index a659c55..f54fdde 100644
--- a/src/fasttext.cc
+++ b/src/fasttext.cc
@@ -86,6 +86,7 @@ namespace fasttext
 
     input_ = std::dynamic_pointer_cast<Matrix>(inputMatrix);
     output_ = std::dynamic_pointer_cast<Matrix>(outputMatrix);
+    quant_ = false;
     wordVectors_.reset();
     args_->dim = input_->size(1);
 
@@ -306,9 +307,9 @@ namespace fasttext
 
     bool quant_input;
     in.read((char *)&quant_input, sizeof(bool));
+    quant_ = quant_input;
     if (quant_input)
     {
-      quant_ = true;
       input_ = std::make_shared<QuantMatrix>();
     }
     input_->load(in);
@@ -386,6 +387,12 @@ namespace fasttext
 
   void FastText::quantize(const Args &qargs, const TrainCallback &callback)
   {
+    if (quant_)
+    {
+      throw std::invalid_argument(
+          "Model is already quantized. "
+          "Quantize the original (non-quantized) model instead.");
+    }
     if (args_->model != model_name::sup)
     {
       throw std::invalid_argument(