Fixed a bug: Wrong save of a preset if the preset is already exist.

Step to repro:
1. Select system "Some_preset" preset
2. Make some changes
3. Save preset with "Some_preset - Copy" name.
=>  "Some_preset - Copy" is selected, there is/are marked option(s) only different from system preset.
repeate 1-3 steps
=>  "Some_preset - Copy" is selected, there is/are marked option(s) different from DEFAULT preset.

src/slic3r/GUI/Preset.cpp

@@ -819,6 +819,21 @@ bool PresetCollection::delete_current_preset()
     return true;
 }
 
+bool PresetCollection::delete_preset(const std::string& name)
+{
+    auto it = this->find_preset_internal(name);
+
+    const Preset& preset = *it;
+    if (preset.is_default)
+        return false;
+    if (!preset.is_external && !preset.is_system) {
+        // Erase the preset file.
+        boost::nowide::remove(preset.file.c_str());
+    }
+    m_presets.erase(it);
+    return true;
+}
+
 void PresetCollection::load_bitmap_default(wxWindow *window, const std::string &file_name)
 {
     // XXX: See note in PresetBundle::load_compatible_bitmaps()

src/slic3r/GUI/Preset.hpp

@@ -276,6 +276,9 @@ public:
     // Delete the current preset, activate the first visible preset.
     // returns true if the preset was deleted successfully.
     bool            delete_current_preset();
+    // Delete the current preset, activate the first visible preset.
+    // returns true if the preset was deleted successfully.
+    bool            delete_preset(const std::string& name);
 
     // Load default bitmap to be placed at the wxBitmapComboBox of a MainFrame.
     void            load_bitmap_default(wxWindow *window, const std::string &file_name);

src/slic3r/GUI/Tab.cpp

@@ -3019,6 +3019,18 @@ void Tab::save_preset(std::string name /*= ""*/)
             show_error(this, _(L("Cannot overwrite an external profile.")));
             return;
         }
+        if (existing && name != preset.name)
+        {
+            wxString msg_text = GUI::from_u8((boost::format(_utf8(L("Preset with name \"%1%\" already exist."))) % name).str());
+            msg_text += "\n" + _(L("Replace?"));
+            wxMessageDialog dialog(nullptr, msg_text, _(L("Warning")), wxICON_WARNING | wxYES | wxNO);
+
+            if (dialog.ShowModal() == wxID_NO)
+                return;
+
+            // Remove the preset from the list.
+            m_presets->delete_preset(name);
+        }
     }
 
     // Save the preset into Slic3r::data_dir / presets / section_name / preset_name.ini