Find why npm dedupe hangs infinitely - npm

Question

I'm trying to flatten my dependency tree. When I'm using command
npm dedupe
It just hangs eating processor and even after half an hour it does not respond...
When I execute npm -ddd dedupe to see what is happening (-ddd sets loglevel to silly level most verbosive possible)
I see that it hangs after building ideal tree:
npm sill idealTree ├─┬ yargs-parser#5.0.0-security.0
npm sill idealTree │ └── camelcase#3.0.0
npm sill idealTree ├─┬ yargs#7.1.1
npm sill idealTree │ └── camelcase#3.0.0
npm sill idealTree └── yup#0.31.0
npm sill dedupe generateActionsToTake
And it hangs on npm sill dedupe generateActionsToTake.
What can be reason of this? npm doctor says this:
Check Value Recommendation
npm ping ok
npm -v v6.14.9
node -v v14.15.1
npm config get registry https://registry.npmjs.org/
which git /usr/bin/git
Perms check on cached files ok
Perms check on global node_modules ok
Perms check on local node_modules ok
Verify cache contents verified 10269 tarballs
I don't know what else should I check. I noticed also that this is happening only in one of my project.