diff --git a/lib/fbe/if_absent.rb b/lib/fbe/if_absent.rb
index 0141b000..1a205e78 100644
--- a/lib/fbe/if_absent.rb
+++ b/lib/fbe/if_absent.rb
@@ -58,7 +58,8 @@ def Fbe.if_absent(fb: Fbe.fb, always: false)
         k = k[0..-2].to_sym
         v = args[1]
         raise(Fbe::Error, "Can't set #{k} to nil") if v.nil?
-        raise(Fbe::Error, "Can't set #{k} to empty string") if v.is_a?(String) && v.empty?
+        raise(Fbe::Error, "Can't set #{k} to empty string") if v == ''
+        raise(Fbe::Error, "Can't set #{k} to #{v.inspect}, it is already #{@map[k].inspect}") if @map.key?(k)
         @map[k] = v
       else
         @map[k.to_sym]
diff --git a/lib/fbe/just_one.rb b/lib/fbe/just_one.rb
index 9465834a..203fedbc 100644
--- a/lib/fbe/just_one.rb
+++ b/lib/fbe/just_one.rb
@@ -43,8 +43,10 @@ def Fbe.just_one(fb: Fbe.fb)
       if k.end_with?('=')
         v = args[1]
         raise(Fbe::Error, "Can't set #{k[0..-2]} to nil") if v.nil?
-        raise(Fbe::Error, "Can't set #{k[0..-2]} to empty string") if v.is_a?(String) && v.empty?
-        @map[k[0..-2].to_sym] = v
+        raise(Fbe::Error, "Can't set #{k[0..-2]} to empty string") if v == ''
+        k = k[0..-2].to_sym
+        raise(Fbe::Error, "Can't set #{k} to #{v.inspect}, it is already #{@map[k].inspect}") if @map.key?(k)
+        @map[k] = v
       else
         @map[k.to_sym]
       end
diff --git a/test/fbe/test_if_absent.rb b/test/fbe/test_if_absent.rb
index 08654c2a..de04e83a 100644
--- a/test/fbe/test_if_absent.rb
+++ b/test/fbe/test_if_absent.rb
@@ -147,4 +147,30 @@ def test_raises_without_block
     assert_equal('A block is required by if_absent', error.message)
     assert_equal(0, fb.size, 'if_absent inserted a fact without a block')
   end
+
+  def test_cannot_set_property_twice
+    seed = Random.new_seed
+    tag = "tæg#{Random.new(seed).rand(1_000_000)}"
+    assert_raises(Fbe::Error, "second value of a property is accepted, seed #{seed}") do
+      Fbe.if_absent(fb: Factbase.new) do |f|
+        f.tag = tag
+        f.tag = "#{tag}ü"
+      end
+    end
+  end
+
+  def test_dont_insert_fact_when_property_is_set_twice
+    seed = Random.new_seed
+    tag = "tæg#{Random.new(seed).rand(1_000_000)}"
+    fb = Factbase.new
+    begin
+      Fbe.if_absent(fb:) do |f|
+        f.tag = tag
+        f.tag = tag
+      end
+    rescue Fbe::Error
+      nil
+    end
+    assert_equal(0, fb.size, "a fact is inserted for a property set twice, seed #{seed}")
+  end
 end
diff --git a/test/fbe/test_just_one.rb b/test/fbe/test_just_one.rb
index 7aaac15a..8c23e43c 100644
--- a/test/fbe/test_just_one.rb
+++ b/test/fbe/test_just_one.rb
@@ -88,4 +88,30 @@ def test_dont_insert_fact_without_block
     end
     assert_equal(0, fb.size, 'just_one inserted a fact without a block')
   end
+
+  def test_cannot_set_property_twice
+    seed = Random.new_seed
+    tag = "tæg#{Random.new(seed).rand(1_000_000)}"
+    assert_raises(Fbe::Error, "second value of a property is accepted, seed #{seed}") do
+      Fbe.just_one(fb: Factbase.new) do |f|
+        f.tag = tag
+        f.tag = "#{tag}ü"
+      end
+    end
+  end
+
+  def test_dont_insert_fact_when_property_is_set_twice
+    seed = Random.new_seed
+    tag = "tæg#{Random.new(seed).rand(1_000_000)}"
+    fb = Factbase.new
+    begin
+      Fbe.just_one(fb:) do |f|
+        f.tag = tag
+        f.tag = tag
+      end
+    rescue Fbe::Error
+      nil
+    end
+    assert_equal(0, fb.size, "a fact is inserted for a property set twice, seed #{seed}")
+  end
 end