Do two hormones have the same effect on a cell if the second messenger is the same?
Secondary signaling molecules like cAMP, Calcium, and Ras-Raf are common in many pathways and I think its a way of integrating different signals.We cannot say that at a moment only one pathway operates in a cell or a single pathway will be enough for a process to happen.I think common second messengers is a resource and time-saving method adapted by our system to coordinate different response efficiently.https://www.nature.com/scitable/topicpage/cell-signaling-14047077

Then how a cell can identify which pathway and process should go on? I think that each time this second messenger will make a small change in choosing down stream effectors.Like Calcium binds to calmodulin which in turn choose next binding partner.Maybe the number of second messengers like concentration and affinity decide which downstream effector will be appropriate for the required stimuli.You can see in this link about how different secondary messengers make different results https://www.ncbi.nlm.nih.gov/books/NBK9870/ 

About GPCR, more than 800 GPCR are there and you can see from this link http://jcs.biologists.org/content/116/24/4867

So, Do two hormones have the same effect on a cell if the second messenger is the same?
I think yes, see this table (https://www.ncbi.nlm.nih.gov/books/NBK21705/)


Here, from the table, you can see adrenaline and glucagon are having the same effect in the liver cell and both have cAMP as a second messenger.But in cardiac muscle and intestine, adrenaline is having a different function. So I think it depends on the cell also.