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I'm using OpenJDK 11 and OpenJFX 11.0.1 with Gradle in IntelliJ IDEA for my project.
When I use javafx.base module, IntelliJ IDEA reports an error at line 1 (project module declaration) saying I'm not including the javafx.beans module (beans is a package in javafx.base module) in my module requirements.
Gradle jar task executes without any errors or warnings so this error is likely related to IntelliJ IDEA.

Error message: "Module 'hr.caellian.lunar' reads package 'javafx.beans' for both 'javafx.base' and 'javafx.base'"
If I remove 'javafx.base' requirement, 'javafx.base' from above error is replaced with the next javafx module.

I'm also getting a warnings at javafx module requirement lines: "Ambiguous module reference: javafx.base" for 'javafx.base' and so on for each javafx module.

Here's my module-info.java file:

module hr.caellian.lunar 
 requires java.base;
 requires kotlin.stdlib;

 requires javafx.base;
 requires javafx.controls;
 requires javafx.fxml;
 opens hr.caellian.lunar.gui to javafx.graphics;

 requires org.kordamp.ikonli.javafx;
 requires org.kordamp.iconli.core;

 requires com.fasterxml.jackson.core;
 requires com.fasterxml.jackson.databind;
 requires com.fasterxml.jackson.dataformat.yaml;

 requires arglib;
 requires org.apache.bcel;

I looked online for a solution for at least half an hour now. All I found was a website saying only the modules I already included are necessary for JFX (which was for JavaFX 9 I think).

I redownloaded JavaFX modules to see if I missed copying something but I didn't - javafx.beans module doesn't exist.

'java --list-modules' doesn't list any javafx modules if that's a problem, although I gathered that it should only list jdk modules and OpenJFX is no longer included with OpenJDK.

I installed OpenJFX SDK into JDK folder (lib files copied to lib folder, same for legal), added javafx jar dependency files from lib folder of JDK11 in IDEA manually. I also added all jmod files to jmod folder of JDK11.

Is it possible JavaFX modules have bad module-info.java files or something like that? Is this IntelliJ IDEA bug?

Managed to fix the problem thanks to the comment from @nullpointer.

The problem was that I had copied files from JavaFX SDK to both my JDK 11 classpath and project lib folder.

If you're seeing this error with the same warnings that means you probably have duplicate module jars (maybe in future modules directly) in your IntelliJ project JDK Classpath and in your build.gradle (or lib folder).

How to fix:
I removed OpenJFX SDK jars from JDK11 Classpath in IntelliJ IDEA in Project Structure. As I deleted JFX SDK jars from my project lib folder, I readded them. Gradle adds every jar in lib folder with:

implementation fileTree(dir: 'lib', include: '**/*.jar')

Also, do not install JavaFX directly into JDK folder. It'll be impossible to update either without keeping unnecessary files, you'll never be able to find all JFX files to remove in case you want yo remove it, JFX src.zip replaces JDK src.zip in lib folder. They're meant to be separate.