As a general rule, prior consistent statements of a witness are inadmissible. There are two primary justifications for the exclusion of such statements: first, they lack probative value (they are self-serving, easily fabricated, and redundant) and second, they constitute hearsay when adduced for the truth of their contents.
R. v. Dinardo, 2008 SCC 24 (CanLII) at para. 36.
Further, their repetition before the trier of fact is capable of working significant prejudice.
See R. v. M.P., 2018 ONCA 608 , at para. 77.
Exceptions to the rule
Like other admissibility rules which are primarily exclusionary in their effect, the general rule enjoining introduction of prior consistent statements of a witness brooks exception.
These exceptions permit introduction of prior consistent statements for restricted purposes which differ depending on the exception, for instance:
1. To rebut a claim of recent fabrication.
R. v. Evans,  2 S.C.R. 629, at p. 643.
2.To provide context in which to assess attacks on testimonial reliability based on alleged prior inconsistencies.
R. v. O.(L.), 2015 ONCA 394, 324 C.C.C. (3d) 562, at para. 34.
3. As narrative.
R. v. F.(J.E.) (1993), 85 C.C.C. (3d) 457 (Ont. C.A.), at p. 476; Khan, at paras. 29-30
A Prior Consistent Statement may be Admissible as Narrative
The fact of a prior complaint may be admissible under the narrative exception to the general rule.
To qualify as narrative, the witness must recount relevant and essential facts which describe and explain his or her experience as a victim of the crime alleged so that the trier of fact will be in a position to understand what happened and how the matter came to the attention of the proper authorities. In all cases where evidence is admitted under the rubric of prior consistent statements, the trial judge is obliged to instruct the jury as to the limited value of the evidence.
The fact that the statements were made is admissible to assist the jury as to the sequence of events from the alleged offence to the prosecution so that they can understand the conduct of the complainant and assess his/her truthfulness.
R. v. Fair, 1993 CanLII 3384 (ON CA).
Limiting instruction to Jury
The jury must be instructed that they are not to look to the content of the statements as proof that a crime has been committed.
However, the effect of the failure of a trial judge to properly apprise a jury about the limited use of prior consistent statements in reaching its verdict varies. Sometimes fatal. Other times, not.
See R. v. M.P., 2018 ONCA 608, at para. 80.