IllegalStateException: alias has more than one write index

I'm getting the same error on all ES data nodes saying that alias has more than one write index so none of data nodes can actually start, cluster is dead. Is there any way to fix the problem without deleting all data? Since all nodes cannot start I cannot use API to fix alias. Loosing data is a bad option.
We use Elastic Search 6.6.1 and ILM

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(Apologies for the original lack of response.)

Original error would have formatted

alias [x] has more than one write index [y,z]

For future note when searched, this is now answered in the blog Troubleshooting ILM blog with the direct summary quote:

When you run Get Aliases, you’ll notice that two indices are marked as is_write_index:true when only one should be per alias. You’ll want to toggle is_write_index:false on one of the indices via the Aliases API.