As a German native speaker I understand the German sentence "Ich werde es für mich behalten." as referring to a secret or other kind of information I promise not to tell anyone.
It took a while until I had an idea how I would say "I will keep this thing for myself" in German: "Ich werde es für mich selbst/selber/persönlich behalten/aufbewahren." With "selbst" , "selber" or "persönlich" it is not suitable for a secret. You can also say: "Ich werde es selbst/selber benutzen." - "I will use it myself." or "Ich nehme es selbst/selber"-"I take is myself." or "Ich nehme es für mich selbst/selber." - "I take it for myself."
P.S.: Before you ask: "selbst" is a bit more formal and "selber" is a bit more informal. Both mean the same.
I guess that in right context it can have both meanings. Check example number five, it has similar construction like Duo sentence...