I understand your worries and concerns! Sorry you got caught up in this snafu. Because I can see that you tried to correct the problem and do the right thing, I hate to worry you. But, maybe this will help you in similar situations in the future.
A nurse can ONLY make judgments about things that fall within her scope of practice. That means that you can ONLY do things that do NOT require any other collaboration with any other heathcare provider. You can only practice and make decisions about things that you can do INDEPENDENTLY.
When not sure if a med has been given, the actions that you can take would be to do an assessment including objective (what the patient and family says) and subjective data (physical and verifiable -- repeatable, things like the BP). That's as far as you can go legally, because as a nursing judgment you are allowed to withhold a medication, but a nursing judgment does NOT allow you to give ANOTHER dose.
After your assessment of the situation, when there is conflicting data, you are required by law to VERIFY AND VALIDATE YOUR DATA. You did that with the other nurse, but she indicated that the med had been given, which supported her documentation.
Your action to validate the data did not verify the data! The data still conflicts, because of your physical assessment and the patient's report. At that point, you were UNABLE TO VERIFY THE DATA. So, you are then obligated legally to resolve the matter by reporting it to the doctor and filling out an incident report for the facility/hospital.
Suppose that the other nurse had really given the glucophage and BP med, and you gave another dose. This would be a med error and could result in a negative patient outcome.
To keep you and your patient safe, remember that ALL conflicting data MUST be verified and validated. ANY nursing action you take, can only be done if the data can be validated COMPLETELY to support your actions. If you are unable to completely and compelingly verify, then you must notify the physician.
Under the ANA's Professional Standards of Practice, you did not successfully "validate" your data. So, you have no legal grounds for giving another dose of the medications. Then, you took a nursing action that required "collaboration" or another healthcare provider (the doctor), by giving a second dose of a med that had been documented as given, and so went outside your scope of practice. And you could be held legally responsible.
I realize that you did not believe the nurse when she said that she had given the meds, and you believed your patient instead. But, in that situation, you put you and your patient at risk by deciding on your own who to believe. A "belief" is not enough justification for risking a med error.
I hope that helps, but I'm afraid I probably just made you feel worse. I've tried to give you the supporting nursing "critical thinking" and supporting basis for this so it didn't sound like I was just fussing at you. I sure do understand the dilemma. Maybe what I've said, though, can help you avoid a problem in the future. Good luck to you. I sure do sympathize with the position you were put in by the other nurse! That nurse needs her license looked at!