According to ImGui API ImGui::IsMouseDoubleClicked(...) call should return true if a mouse was double-clicked. In practice, it always returns false. Only SFML method sf::Mouse::isMouseButtonPressed(...), which scans for single clicks, works. However, it is not useful as there is no built-in method to check for double clicking in SFML Mouse object.
if (ImGui::IsMouseDoubleClicked(0)) { std::cout << "Mouse was double-clicked\n"; }