Android build failed error

When I build for Android, it builds almost entirely and then gives me this error at the very end:

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java.lang.RuntimeException:
java.lang.RuntimeException:
com.android.builder.dexing.DexArchiveMergerException:
Unable to merge dex See the Console for details.

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The unable to merge dex means nothing to me, so I hoped to gain meaning out of the Console. Unfortunately, the stuff in the Console is all Greek to me.

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UnityEditor.BuildPlayerWindow+BuildMethodException: 2 errors
at UnityEditor.BuildPlayerWindow+DefaultBuildMethods.BuildPlayer (BuildPlayerOptions options) [0x0021f] in C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindowBuildMethods.cs:187
at UnityEditor.BuildPlayerWindow.CallBuildMethods (Boolean askForBuildLocation, BuildOptions defaultBuildOptions) [0x0007f] in C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindowBuildMethods.cs:94
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

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Does anyone have any idea what this might mean, and how to fix it?

Make a gradle build.