[build][test] MultiProjectTests.testCycle* are unstable

In I Build tests [this link may vanish later]: https://download.eclipse.org/eclipse/downloads/drops4/I20241014-0530/testresults/html/org.eclipse.jdt.core.tests.builder_ep434I-unit-win32-java17_win32.win32.x86_64_17.html

(1)
org.eclipse.jdt.core.tests.builder.MultiProjectTests | testCycle7| Error

Build path contains duplicate entry: 'src' for project 'P1'

Java Model Exception: Error in Java Model (code 977): Build path contains duplicate entry: 'src' for project 'P1'
at org.eclipse.jdt.internal.core.JavaModelOperation.runOperation(JavaModelOperation.java:812)
at org.eclipse.jdt.internal.core.JavaProject.setRawClasspath(JavaProject.java:3494)
at org.eclipse.jdt.internal.core.JavaProject.setRawClasspath(JavaProject.java:3454)
at org.eclipse.jdt.internal.core.JavaProject.setRawClasspath(JavaProject.java:3510)
at org.eclipse.jdt.core.tests.builder.TestingEnvironment.setClasspath(TestingEnvironment.java:1179)
at org.eclipse.jdt.core.tests.builder.TestingEnvironment.addEntry(TestingEnvironment.java:365)
at org.eclipse.jdt.core.tests.builder.TestingEnvironment.addPackageFragmentRoot(TestingEnvironment.java:191)
at org.eclipse.jdt.core.tests.builder.TestingEnvironment.addPackageFragmentRoot(TestingEnvironment.java:169)
at org.eclipse.jdt.core.tests.builder.TestingEnvironment.addPackageFragmentRoot(TestingEnvironment.java:165)
at org.eclipse.jdt.core.tests.builder.TestingEnvironment.addPackageFragmentRoot(TestingEnvironment.java:152)
at org.eclipse.jdt.core.tests.builder.MultiProjectTests.testCycle7(MultiProjectTests.java:1100)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at junit.framework.TestCase.runTest(TestCase.java:177)
at org.eclipse.jdt.core.tests.junit.extension.TestCase.runTest(TestCase.java:970)
at junit.framework.TestCase.runBare(TestCase.java:142)
at junit.framework.TestResult$1.protect(TestResult.java:122)
at junit.framework.TestResult.runProtected(TestResult.java:142)
at junit.framework.TestResult.run(TestResult.java:125)
at junit.framework.TestCase.run(TestCase.java:130)
at junit.framework.TestSuite.runTest(TestSuite.java:241)
at junit.framework.TestSuite.run(TestSuite.java:236)
at junit.framework.TestSuite.runTest(TestSuite.java:241)
at junit.framework.TestSuite.run(TestSuite.java:236)

(2)

org.eclipse.jdt.core.tests.builder.MultiProjectTests | testCycle8 | Error |

Build path contains duplicate entry: 'P2' for project 'P1'Java Model Exception: Error in Java Model (code 977): Build path contains duplicate entry: 'P2' for project 'P1'at

org.eclipse.jdt.internal.core.JavaModelOperation.runOperation(JavaModelOperation.java:812)at org.eclipse.jdt.internal.core.JavaProject.setRawClasspath(JavaProject.java:3494)at org.eclipse.jdt.internal.core.JavaProject.setRawClasspath(JavaProject.java:3454)at org.eclipse.jdt.internal.core.JavaProject.setRawClasspath(JavaProject.java:3510)at org.eclipse.jdt.core.tests.builder.TestingEnvironment.setClasspath(TestingEnvironment.java:1179)at org.eclipse.jdt.core.tests.builder.TestingEnvironment.addEntry(TestingEnvironment.java:365)at org.eclipse.jdt.core.tests.builder.TestingEnvironment.addRequiredProject(TestingEnvironment.java:296)at org.eclipse.jdt.core.tests.builder.MultiProjectTests.testCycle8(MultiProjectTests.java:1210)at java.base/java.lang.reflect.Method.invoke(Method.java:568)at junit.framework.TestCase.runTest(TestCase.java:177)at org.eclipse.jdt.core.tests.junit.extension.TestCase.runTest(TestCase.java:970)at junit.framework.TestCase.runBare(TestCase.java:142)at junit.framework.TestResult$1.protect(TestResult.java:122)at junit.framework.TestResult.runProtected(TestResult.java:142)at junit.framework.TestResult.run(TestResult.java:125)at junit.framework.TestCase.run(TestCase.java:130)at junit.framework.TestSuite.runTest(TestSuite.java:241)at junit.framework.TestSuite.run(TestSuite.java:236)at junit.framework.TestSuite.runTest(TestSuite.java:241)at junit.framework.TestSuite.run(TestSuite.java:236)
-- | -- | -- | --

Please take a look at similar issues from bugzilla:

https://bugs.eclipse.org/bugs/show_bug.cgi?id=412484
https://bugs.eclipse.org/bugs/show_bug.cgi?id=531897
https://bugs.eclipse.org/bugs/show_bug.cgi?id=505822
https://bugs.eclipse.org/bugs/show_bug.cgi?id=162054

Start with MultiProjectTests.java at testCycle7 (line 1100) and testCycle8 (line 1210), then follow TestingEnvironment.addPackageFragmentRoot, addRequiredProject, and setClasspath from the reported stack traces. Reproduce the instability in the builder test suite and determine what is needed for these tests to complete without duplicate classpath-entry errors.

Written by the indexing model from the issue text.