Anecdote: I had a colleague which hired a (full-time) worker for an agreed upon annual salary... two weeks into the employment agreement this worker asked for a raise which was apparently substantial... he was fired.
You can ruin a relationship by being greedy.
Client A paid you to learn this "component". They invested in you so you could further be of service to them. Without that investment you may not know this "component". In addition, without this knowledge you may not be getting any additional work from the client.
Now, you want to charge this same client a premium for the knowledge they paid you to learn. They already paid a premium with that first project taking 2 months rather than 2 days.
This is merely bad practice.
Charge the next client needing this "component" a premium for the knowledge/experience. However, never charge the client that invested in you to begin with and supplied you with this knowledge.
Look at it another way.. via physical objects....
You have nothing to sit on... So, Bob paid for a new office chair so you could work on his project. Now, Bob has a second project. You want to charge Bob for office chair maintenance since you have a new one you need to upkeep. Considering Bob paid for the chair initially, doesn't that sound silly and greedy?
I have a collection of past items I've used and own (personal). I also have a collection of past items I've used for each client (project).
- If I pull something from the "personal" collection, I charge a markup for it's use.
- If I pull something from the "project' collection, that client is never charged a markup for its use. They paid for it's creation (even if I technically own the item IP according to contracts.)
- I don't pull from Client A "projects" for use in Client B "projects". Anything which is "free to use anywhere", i.e. created on my own time generally or any associated client relationship has long since been dissolved, is in the "personal" collection.
Of course, this is all highly dependent upon the specialization and/or generalization of any assets and any associated contracts.