Map.containsKey call is sometimes unnecessary, when it's known that Map doesn't contain null values.
Instead of pair containsKey+put we can use putIfAbsent and compare result with null.
I found 4 such cases in ModuleDescriptor.Builder:
1. java.lang.module.ModuleDescriptor.Builder#requires(java.lang.module.ModuleDescriptor.Requires)

            String mn = req.name();
            if (name.equals(mn))
                throw new IllegalArgumentException("Dependence on self");
            if (requires.containsKey(mn))
                throw new IllegalStateException("Dependence upon " + mn
                                                + " already declared");
            requires.put(mn, req);

2. java.lang.module.ModuleDescriptor.Builder#exports(java.lang.module.ModuleDescriptor.Exports)

            String source = e.source();
            if (exports.containsKey(source)) {
                throw new IllegalStateException("Exported package " + source
                                                 + " already declared");
            }
            exports.put(source, e);

3. java.lang.module.ModuleDescriptor.Builder#opens(java.lang.module.ModuleDescriptor.Opens)

            String source = obj.source();
            if (opens.containsKey(source)) {
                throw new IllegalStateException("Open package " + source
                                                + " already declared");
            }
            opens.put(source, obj);

4. java.lang.module.ModuleDescriptor.Builder#provides(java.lang.module.ModuleDescriptor.Provides)

            String service = p.service();
            if (provides.containsKey(service))
                throw new IllegalStateException("Providers of service "
                                                + service + " already declared");
            provides.put(service, p);