Answer:
No, there are multiple ways in which different mutations in the same gene can cause the same phenotype
Step-by-step explanation:
Several different mechanisms of mutation can lead to the same phenotype. For example, lets say our phenotype is that flies have white eyes, and we know that this occurs in one particular gene that normally makes the eye colour red. (the red gene)
These mutations likely rendered the red gene ineffective (as the eyes are not red). However, this could happen in a variety of ways.
There could be a single base deletion in the first exon of the mRNA, changing the reading frame of the protein and messing up the entire sequence (a frame shift mutations)
The entire gene could be deleted
A single base could be substituted in an important site of the gene, for example, one which translates into a catalytic residue or binding site in the protein
There could be an inversion at the promoter region of the gene, such that a transcription factor can no longer bind to transcribe the gene.
There are countless other ways in which a mutation could have been caused. Therefore, just because we know the same gene is affected does not mean that we can assume the mutations are identical.